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7 <chapter id="drd-manual" xreflabel="DRD: a thread error detector">
8 <title>DRD: a thread error detector</title>
10 <para>To use this tool, you must specify
11 <option>--tool=drd</option>
12 on the Valgrind command line.</para>
15 <sect1 id="drd-manual.overview" xreflabel="Overview">
16 <title>Overview</title>
18 <para>
19 DRD is a Valgrind tool for detecting errors in multithreaded C and C++
20 programs. The tool works for any program that uses the POSIX threading
21 primitives or that uses threading concepts built on top of the POSIX threading
22 primitives.
23 </para>
25 <sect2 id="drd-manual.mt-progr-models" xreflabel="MT-progr-models">
26 <title>Multithreaded Programming Paradigms</title>
28 <para>
29 There are two possible reasons for using multithreading in a program:
30 <itemizedlist>
31 <listitem>
32 <para>
33 To model concurrent activities. Assigning one thread to each activity
34 can be a great simplification compared to multiplexing the states of
35 multiple activities in a single thread. This is why most server software
36 and embedded software is multithreaded.
37 </para>
38 </listitem>
39 <listitem>
40 <para>
41 To use multiple CPU cores simultaneously for speeding up
42 computations. This is why many High Performance Computing (HPC)
43 applications are multithreaded.
44 </para>
45 </listitem>
46 </itemizedlist>
47 </para>
49 <para>
50 Multithreaded programs can use one or more of the following programming
51 paradigms. Which paradigm is appropriate depends e.g. on the application type.
52 Some examples of multithreaded programming paradigms are:
53 <itemizedlist>
54 <listitem>
55 <para>
56 Locking. Data that is shared over threads is protected from concurrent
57 accesses via locking. E.g. the POSIX threads library, the Qt library
58 and the Boost.Thread library support this paradigm directly.
59 </para>
60 </listitem>
61 <listitem>
62 <para>
63 Message passing. No data is shared between threads, but threads exchange
64 data by passing messages to each other. Examples of implementations of
65 the message passing paradigm are MPI and CORBA.
66 </para>
67 </listitem>
68 <listitem>
69 <para>
70 Automatic parallelization. A compiler converts a sequential program into
71 a multithreaded program. The original program may or may not contain
72 parallelization hints. One example of such parallelization hints is the
73 OpenMP standard. In this standard a set of directives are defined which
74 tell a compiler how to parallelize a C, C++ or Fortran program. OpenMP
75 is well suited for computational intensive applications. As an example,
76 an open source image processing software package is using OpenMP to
77 maximize performance on systems with multiple CPU
78 cores. GCC supports the
79 OpenMP standard from version 4.2.0 on.
80 </para>
81 </listitem>
82 <listitem>
83 <para>
84 Software Transactional Memory (STM). Any data that is shared between
85 threads is updated via transactions. After each transaction it is
86 verified whether there were any conflicting transactions. If there were
87 conflicts, the transaction is aborted, otherwise it is committed. This
88 is a so-called optimistic approach. There is a prototype of the Intel C++
89 Compiler available that supports STM. Research about the addition of
90 STM support to GCC is ongoing.
91 </para>
92 </listitem>
93 </itemizedlist>
94 </para>
96 <para>
97 DRD supports any combination of multithreaded programming paradigms as
98 long as the implementation of these paradigms is based on the POSIX
99 threads primitives. DRD however does not support programs that use
100 e.g. Linux' futexes directly. Attempts to analyze such programs with
101 DRD will cause DRD to report many false positives.
102 </para>
104 </sect2>
107 <sect2 id="drd-manual.pthreads-model" xreflabel="Pthreads-model">
108 <title>POSIX Threads Programming Model</title>
110 <para>
111 POSIX threads, also known as Pthreads, is the most widely available
112 threading library on Unix systems.
113 </para>
115 <para>
116 The POSIX threads programming model is based on the following abstractions:
117 <itemizedlist>
118 <listitem>
119 <para>
120 A shared address space. All threads running within the same
121 process share the same address space. All data, whether shared or
122 not, is identified by its address.
123 </para>
124 </listitem>
125 <listitem>
126 <para>
127 Regular load and store operations, which allow to read values
128 from or to write values to the memory shared by all threads
129 running in the same process.
130 </para>
131 </listitem>
132 <listitem>
133 <para>
134 Atomic store and load-modify-store operations. While these are
135 not mentioned in the POSIX threads standard, most
136 microprocessors support atomic memory operations.
137 </para>
138 </listitem>
139 <listitem>
140 <para>
141 Threads. Each thread represents a concurrent activity.
142 </para>
143 </listitem>
144 <listitem>
145 <para>
146 Synchronization objects and operations on these synchronization
147 objects. The following types of synchronization objects have been
148 defined in the POSIX threads standard: mutexes, condition variables,
149 semaphores, reader-writer synchronization objects, barriers and
150 spinlocks.
151 </para>
152 </listitem>
153 </itemizedlist>
154 </para>
156 <para>
157 Which source code statements generate which memory accesses depends on
158 the <emphasis>memory model</emphasis> of the programming language being
159 used. There is not yet a definitive memory model for the C and C++
160 languages. For a draft memory model, see also the document
161 <ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html">
162 WG21/N2338: Concurrency memory model compiler consequences</ulink>.
163 </para>
165 <para>
166 For more information about POSIX threads, see also the Single UNIX
167 Specification version 3, also known as
168 <ulink url="http://www.opengroup.org/onlinepubs/000095399/idx/threads.html">
169 IEEE Std 1003.1</ulink>.
170 </para>
172 </sect2>
175 <sect2 id="drd-manual.mt-problems" xreflabel="MT-Problems">
176 <title>Multithreaded Programming Problems</title>
178 <para>
179 Depending on which multithreading paradigm is being used in a program,
180 one or more of the following problems can occur:
181 <itemizedlist>
182 <listitem>
183 <para>
184 Data races. One or more threads access the same memory location without
185 sufficient locking. Most but not all data races are programming errors
186 and are the cause of subtle and hard-to-find bugs.
187 </para>
188 </listitem>
189 <listitem>
190 <para>
191 Lock contention. One thread blocks the progress of one or more other
192 threads by holding a lock too long.
193 </para>
194 </listitem>
195 <listitem>
196 <para>
197 Improper use of the POSIX threads API. Most implementations of the POSIX
198 threads API have been optimized for runtime speed. Such implementations
199 will not complain on certain errors, e.g. when a mutex is being unlocked
200 by another thread than the thread that obtained a lock on the mutex.
201 </para>
202 </listitem>
203 <listitem>
204 <para>
205 Deadlock. A deadlock occurs when two or more threads wait for
206 each other indefinitely.
207 </para>
208 </listitem>
209 <listitem>
210 <para>
211 False sharing. If threads that run on different processor cores
212 access different variables located in the same cache line
213 frequently, this will slow down the involved threads a lot due
214 to frequent exchange of cache lines.
215 </para>
216 </listitem>
217 </itemizedlist>
218 </para>
220 <para>
221 Although the likelihood of the occurrence of data races can be reduced
222 through a disciplined programming style, a tool for automatic
223 detection of data races is a necessity when developing multithreaded
224 software. DRD can detect these, as well as lock contention and
225 improper use of the POSIX threads API.
226 </para>
228 </sect2>
231 <sect2 id="drd-manual.data-race-detection" xreflabel="data-race-detection">
232 <title>Data Race Detection</title>
234 <para>
235 The result of load and store operations performed by a multithreaded program
236 depends on the order in which memory operations are performed. This order is
237 determined by:
238 <orderedlist>
239 <listitem>
240 <para>
241 All memory operations performed by the same thread are performed in
242 <emphasis>program order</emphasis>, that is, the order determined by the
243 program source code and the results of previous load operations.
244 </para>
245 </listitem>
246 <listitem>
247 <para>
248 Synchronization operations determine certain ordering constraints on
249 memory operations performed by different threads. These ordering
250 constraints are called the <emphasis>synchronization order</emphasis>.
251 </para>
252 </listitem>
253 </orderedlist>
254 The combination of program order and synchronization order is called the
255 <emphasis>happens-before relationship</emphasis>. This concept was first
256 defined by S. Adve et al in the paper <emphasis>Detecting data races on weak
257 memory systems</emphasis>, ACM SIGARCH Computer Architecture News, v.19 n.3,
258 p.234-243, May 1991.
259 </para>
261 <para>
262 Two memory operations <emphasis>conflict</emphasis> if both operations are
263 performed by different threads, refer to the same memory location and at least
264 one of them is a store operation.
265 </para>
267 <para>
268 A multithreaded program is <emphasis>data-race free</emphasis> if all
269 conflicting memory accesses are ordered by synchronization
270 operations.
271 </para>
273 <para>
274 A well known way to ensure that a multithreaded program is data-race
275 free is to ensure that a locking discipline is followed. It is e.g.
276 possible to associate a mutex with each shared data item, and to hold
277 a lock on the associated mutex while the shared data is accessed.
278 </para>
280 <para>
281 All programs that follow a locking discipline are data-race free, but not all
282 data-race free programs follow a locking discipline. There exist multithreaded
283 programs where access to shared data is arbitrated via condition variables,
284 semaphores or barriers. As an example, a certain class of HPC applications
285 consists of a sequence of computation steps separated in time by barriers, and
286 where these barriers are the only means of synchronization. Although there are
287 many conflicting memory accesses in such applications and although such
288 applications do not make use mutexes, most of these applications do not
289 contain data races.
290 </para>
292 <para>
293 There exist two different approaches for verifying the correctness of
294 multithreaded programs at runtime. The approach of the so-called Eraser
295 algorithm is to verify whether all shared memory accesses follow a consistent
296 locking strategy. And the happens-before data race detectors verify directly
297 whether all interthread memory accesses are ordered by synchronization
298 operations. While the last approach is more complex to implement, and while it
299 is more sensitive to OS scheduling, it is a general approach that works for
300 all classes of multithreaded programs. An important advantage of
301 happens-before data race detectors is that these do not report any false
302 positives.
303 </para>
305 <para>
306 DRD is based on the happens-before algorithm.
307 </para>
309 </sect2>
312 </sect1>
315 <sect1 id="drd-manual.using-drd" xreflabel="Using DRD">
316 <title>Using DRD</title>
318 <sect2 id="drd-manual.options" xreflabel="DRD Command-line Options">
319 <title>DRD Command-line Options</title>
321 <para>The following command-line options are available for controlling the
322 behavior of the DRD tool itself:</para>
324 <!-- start of xi:include in the manpage -->
325 <variablelist id="drd.opts.list">
326 <varlistentry>
327 <term>
328 <option><![CDATA[--check-stack-var=<yes|no> [default: no]]]></option>
329 </term>
330 <listitem>
331 <para>
332 Controls whether DRD detects data races on stack
333 variables. Verifying stack variables is disabled by default because
334 most programs do not share stack variables over threads.
335 </para>
336 </listitem>
337 </varlistentry>
338 <varlistentry>
339 <term>
340 <option><![CDATA[--exclusive-threshold=<n> [default: off]]]></option>
341 </term>
342 <listitem>
343 <para>
344 Print an error message if any mutex or writer lock has been
345 held longer than the time specified in milliseconds. This
346 option enables the detection of lock contention.
347 </para>
348 </listitem>
349 </varlistentry>
350 <varlistentry>
351 <term>
352 <option><![CDATA[--join-list-vol=<n> [default: 10]]]></option>
353 </term>
354 <listitem>
355 <para>
356 Data races that occur between a statement at the end of one thread
357 and another thread can be missed if memory access information is
358 discarded immediately after a thread has been joined. This option
359 allows one to specify for how many joined threads memory access information
360 should be retained.
361 </para>
362 </listitem>
363 </varlistentry>
364 <varlistentry>
365 <term>
366 <option>
367 <![CDATA[--first-race-only=<yes|no> [default: no]]]>
368 </option>
369 </term>
370 <listitem>
371 <para>
372 Whether to report only the first data race that has been detected on a
373 memory location or all data races that have been detected on a memory
374 location.
375 </para>
376 </listitem>
377 </varlistentry>
378 <varlistentry>
379 <term>
380 <option>
381 <![CDATA[--free-is-write=<yes|no> [default: no]]]>
382 </option>
383 </term>
384 <listitem>
385 <para>
386 Whether to report races between accessing memory and freeing
387 memory. Enabling this option may cause DRD to run slightly
388 slower. Notes:</para>
389 <itemizedlist>
390 <listitem>
391 <para>
392 Don't enable this option when using custom memory allocators
393 that use
394 the <computeroutput>VG_USERREQ__MALLOCLIKE_BLOCK</computeroutput>
395 and <computeroutput>VG_USERREQ__FREELIKE_BLOCK</computeroutput>
396 because that would result in false positives.
397 </para>
398 </listitem>
399 <listitem>
400 <para>Don't enable this option when using reference-counted
401 objects because that will result in false positives, even when
402 that code has been annotated properly with
403 <computeroutput>ANNOTATE_HAPPENS_BEFORE</computeroutput>
404 and <computeroutput>ANNOTATE_HAPPENS_AFTER</computeroutput>. See
405 e.g. the output of the following command for an example:
406 <computeroutput>valgrind --tool=drd --free-is-write=yes
407 drd/tests/annotate_smart_pointer</computeroutput>.
408 </para>
409 </listitem>
410 </itemizedlist>
411 </listitem>
412 </varlistentry>
413 <varlistentry>
414 <term>
415 <option>
416 <![CDATA[--report-signal-unlocked=<yes|no> [default: yes]]]>
417 </option>
418 </term>
419 <listitem>
420 <para>
421 Whether to report calls to
422 <function>pthread_cond_signal</function> and
423 <function>pthread_cond_broadcast</function> where the mutex
424 associated with the signal through
425 <function>pthread_cond_wait</function> or
426 <function>pthread_cond_timed_wait</function>is not locked at
427 the time the signal is sent. Sending a signal without holding
428 a lock on the associated mutex is a common programming error
429 which can cause subtle race conditions and unpredictable
430 behavior. There exist some uncommon synchronization patterns
431 however where it is safe to send a signal without holding a
432 lock on the associated mutex.
433 </para>
434 </listitem>
435 </varlistentry>
436 <varlistentry>
437 <term>
438 <option><![CDATA[--segment-merging=<yes|no> [default: yes]]]></option>
439 </term>
440 <listitem>
441 <para>
442 Controls segment merging. Segment merging is an algorithm to
443 limit memory usage of the data race detection
444 algorithm. Disabling segment merging may improve the accuracy
445 of the so-called 'other segments' displayed in race reports
446 but can also trigger an out of memory error.
447 </para>
448 </listitem>
449 </varlistentry>
450 <varlistentry>
451 <term>
452 <option><![CDATA[--segment-merging-interval=<n> [default: 10]]]></option>
453 </term>
454 <listitem>
455 <para>
456 Perform segment merging only after the specified number of new
457 segments have been created. This is an advanced configuration option
458 that allows one to choose whether to minimize DRD's memory usage by
459 choosing a low value or to let DRD run faster by choosing a slightly
460 higher value. The optimal value for this parameter depends on the
461 program being analyzed. The default value works well for most programs.
462 </para>
463 </listitem>
464 </varlistentry>
465 <varlistentry>
466 <term>
467 <option><![CDATA[--shared-threshold=<n> [default: off]]]></option>
468 </term>
469 <listitem>
470 <para>
471 Print an error message if a reader lock has been held longer
472 than the specified time (in milliseconds). This option enables
473 the detection of lock contention.
474 </para>
475 </listitem>
476 </varlistentry>
477 <varlistentry>
478 <term>
479 <option><![CDATA[--show-confl-seg=<yes|no> [default: yes]]]></option>
480 </term>
481 <listitem>
482 <para>
483 Show conflicting segments in race reports. Since this
484 information can help to find the cause of a data race, this
485 option is enabled by default. Disabling this option makes the
486 output of DRD more compact.
487 </para>
488 </listitem>
489 </varlistentry>
490 <varlistentry>
491 <term>
492 <option><![CDATA[--show-stack-usage=<yes|no> [default: no]]]></option>
493 </term>
494 <listitem>
495 <para>
496 Print stack usage at thread exit time. When a program creates a large
497 number of threads it becomes important to limit the amount of virtual
498 memory allocated for thread stacks. This option makes it possible to
499 observe how much stack memory has been used by each thread of the
500 client program. Note: the DRD tool itself allocates some temporary
501 data on the client thread stack. The space necessary for this
502 temporary data must be allocated by the client program when it
503 allocates stack memory, but is not included in stack usage reported by
504 DRD.
505 </para>
506 </listitem>
507 </varlistentry>
508 <varlistentry>
509 <term>
510 <option><![CDATA[--ignore-thread-creation=<yes|no> [default: no]]]></option>
511 </term>
512 <listitem>
513 <para>
514 Controls whether all activities during thread creation should be
515 ignored. By default enabled only on Solaris.
516 Solaris provides higher throughput, parallelism and scalability than
517 other operating systems, at the cost of more fine-grained locking
518 activity. This means for example that when a thread is created under
519 glibc, just one big lock is used for all thread setup. Solaris libc
520 uses several fine-grained locks and the creator thread resumes its
521 activities as soon as possible, leaving for example stack and TLS setup
522 sequence to the created thread.
523 This situation confuses DRD as it assumes there is some false ordering
524 in place between creator and created thread; and therefore many types
525 of race conditions in the application would not be reported. To prevent
526 such false ordering, this command line option is set to
527 <computeroutput>yes</computeroutput> by default on Solaris.
528 All activity (loads, stores, client requests) is therefore ignored
529 during:</para>
530 <itemizedlist>
531 <listitem>
532 <para>
533 pthread_create() call in the creator thread
534 </para>
535 </listitem>
536 <listitem>
537 <para>
538 thread creation phase (stack and TLS setup) in the created thread
539 </para>
540 </listitem>
541 </itemizedlist>
542 </listitem>
543 </varlistentry>
544 </variablelist>
545 <!-- end of xi:include in the manpage -->
547 <!-- start of xi:include in the manpage -->
548 <para>
549 The following options are available for monitoring the behavior of the
550 client program:
551 </para>
553 <variablelist id="drd.debugopts.list">
554 <varlistentry>
555 <term>
556 <option><![CDATA[--trace-addr=<address> [default: none]]]></option>
557 </term>
558 <listitem>
559 <para>
560 Trace all load and store activity for the specified
561 address. This option may be specified more than once.
562 </para>
563 </listitem>
564 </varlistentry>
565 <varlistentry>
566 <term>
567 <option><![CDATA[--ptrace-addr=<address> [default: none]]]></option>
568 </term>
569 <listitem>
570 <para>
571 Trace all load and store activity for the specified address and keep
572 doing that even after the memory at that address has been freed and
573 reallocated.
574 </para>
575 </listitem>
576 </varlistentry>
577 <varlistentry>
578 <term>
579 <option><![CDATA[--trace-alloc=<yes|no> [default: no]]]></option>
580 </term>
581 <listitem>
582 <para>
583 Trace all memory allocations and deallocations. May produce a huge
584 amount of output.
585 </para>
586 </listitem>
587 </varlistentry>
588 <varlistentry>
589 <term>
590 <option><![CDATA[--trace-barrier=<yes|no> [default: no]]]></option>
591 </term>
592 <listitem>
593 <para>
594 Trace all barrier activity.
595 </para>
596 </listitem>
597 </varlistentry>
598 <varlistentry>
599 <term>
600 <option><![CDATA[--trace-cond=<yes|no> [default: no]]]></option>
601 </term>
602 <listitem>
603 <para>
604 Trace all condition variable activity.
605 </para>
606 </listitem>
607 </varlistentry>
608 <varlistentry>
609 <term>
610 <option><![CDATA[--trace-fork-join=<yes|no> [default: no]]]></option>
611 </term>
612 <listitem>
613 <para>
614 Trace all thread creation and all thread termination events.
615 </para>
616 </listitem>
617 </varlistentry>
618 <varlistentry>
619 <term>
620 <option><![CDATA[--trace-hb=<yes|no> [default: no]]]></option>
621 </term>
622 <listitem>
623 <para>
624 Trace execution of the <literal>ANNOTATE_HAPPENS_BEFORE()</literal>,
625 <literal>ANNOTATE_HAPPENS_AFTER()</literal> and
626 <literal>ANNOTATE_HAPPENS_DONE()</literal> client requests.
627 </para>
628 </listitem>
629 </varlistentry>
630 <varlistentry>
631 <term>
632 <option><![CDATA[--trace-mutex=<yes|no> [default: no]]]></option>
633 </term>
634 <listitem>
635 <para>
636 Trace all mutex activity.
637 </para>
638 </listitem>
639 </varlistentry>
640 <varlistentry>
641 <term>
642 <option><![CDATA[--trace-rwlock=<yes|no> [default: no]]]></option>
643 </term>
644 <listitem>
645 <para>
646 Trace all reader-writer lock activity.
647 </para>
648 </listitem>
649 </varlistentry>
650 <varlistentry>
651 <term>
652 <option><![CDATA[--trace-semaphore=<yes|no> [default: no]]]></option>
653 </term>
654 <listitem>
655 <para>
656 Trace all semaphore activity.
657 </para>
658 </listitem>
659 </varlistentry>
660 </variablelist>
661 <!-- end of xi:include in the manpage -->
663 </sect2>
666 <sect2 id="drd-manual.data-races" xreflabel="Data Races">
667 <title>Detected Errors: Data Races</title>
669 <para>
670 DRD prints a message every time it detects a data race. Please keep
671 the following in mind when interpreting DRD's output:
672 <itemizedlist>
673 <listitem>
674 <para>
675 Every thread is assigned a <emphasis>thread ID</emphasis> by the DRD
676 tool. A thread ID is a number. Thread ID's start at one and are never
677 recycled.
678 </para>
679 </listitem>
680 <listitem>
681 <para>
682 The term <emphasis>segment</emphasis> refers to a consecutive
683 sequence of load, store and synchronization operations, all
684 issued by the same thread. A segment always starts and ends at a
685 synchronization operation. Data race analysis is performed
686 between segments instead of between individual load and store
687 operations because of performance reasons.
688 </para>
689 </listitem>
690 <listitem>
691 <para>
692 There are always at least two memory accesses involved in a data
693 race. Memory accesses involved in a data race are called
694 <emphasis>conflicting memory accesses</emphasis>. DRD prints a
695 report for each memory access that conflicts with a past memory
696 access.
697 </para>
698 </listitem>
699 </itemizedlist>
700 </para>
702 <para>
703 Below you can find an example of a message printed by DRD when it
704 detects a data race:
705 </para>
706 <programlisting><![CDATA[
707 $ valgrind --tool=drd --read-var-info=yes drd/tests/rwlock_race
708 ...
709 ==9466== Thread 3:
710 ==9466== Conflicting load by thread 3 at 0x006020b8 size 4
711 ==9466== at 0x400B6C: thread_func (rwlock_race.c:29)
712 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
713 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
714 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
715 ==9466== Location 0x6020b8 is 0 bytes inside local var "s_racy"
716 ==9466== declared at rwlock_race.c:18, in frame #0 of thread 3
717 ==9466== Other segment start (thread 2)
718 ==9466== at 0x4C2847D: pthread_rwlock_rdlock* (drd_pthread_intercepts.c:813)
719 ==9466== by 0x400B6B: thread_func (rwlock_race.c:28)
720 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
721 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
722 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
723 ==9466== Other segment end (thread 2)
724 ==9466== at 0x4C28B54: pthread_rwlock_unlock* (drd_pthread_intercepts.c:912)
725 ==9466== by 0x400B84: thread_func (rwlock_race.c:30)
726 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
727 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
728 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
729 ...
730 ]]></programlisting>
732 <para>
733 The above report has the following meaning:
734 <itemizedlist>
735 <listitem>
736 <para>
737 The number in the column on the left is the process ID of the
738 process being analyzed by DRD.
739 </para>
740 </listitem>
741 <listitem>
742 <para>
743 The first line ("Thread 3") tells you the thread ID for
744 the thread in which context the data race has been detected.
745 </para>
746 </listitem>
747 <listitem>
748 <para>
749 The next line tells which kind of operation was performed (load or
750 store) and by which thread. On the same line the start address and the
751 number of bytes involved in the conflicting access are also displayed.
752 </para>
753 </listitem>
754 <listitem>
755 <para>
756 Next, the call stack of the conflicting access is displayed. If
757 your program has been compiled with debug information
758 (<option>-g</option>), this call stack will include file names and
759 line numbers. The two
760 bottommost frames in this call stack (<function>clone</function>
761 and <function>start_thread</function>) show how the NPTL starts
762 a thread. The third frame
763 (<function>vg_thread_wrapper</function>) is added by DRD. The
764 fourth frame (<function>thread_func</function>) is the first
765 interesting line because it shows the thread entry point, that
766 is the function that has been passed as the third argument to
767 <function>pthread_create</function>.
768 </para>
769 </listitem>
770 <listitem>
771 <para>
772 Next, the allocation context for the conflicting address is
773 displayed. For dynamically allocated data the allocation call
774 stack is shown. For static variables and stack variables the
775 allocation context is only shown when the option
776 <option>--read-var-info=yes</option> has been
777 specified. Otherwise DRD will print <computeroutput>Allocation
778 context: unknown</computeroutput>.
779 </para>
780 </listitem>
781 <listitem>
782 <para>
783 A conflicting access involves at least two memory accesses. For
784 one of these accesses an exact call stack is displayed, and for
785 the other accesses an approximate call stack is displayed,
786 namely the start and the end of the segments of the other
787 accesses. This information can be interpreted as follows:
788 <orderedlist>
789 <listitem>
790 <para>
791 Start at the bottom of both call stacks, and count the
792 number stack frames with identical function name, file
793 name and line number. In the above example the three
794 bottommost frames are identical
795 (<function>clone</function>,
796 <function>start_thread</function> and
797 <function>vg_thread_wrapper</function>).
798 </para>
799 </listitem>
800 <listitem>
801 <para>
802 The next higher stack frame in both call stacks now tells
803 you between in which source code region the other memory
804 access happened. The above output tells that the other
805 memory access involved in the data race happened between
806 source code lines 28 and 30 in file
807 <computeroutput>rwlock_race.c</computeroutput>.
808 </para>
809 </listitem>
810 </orderedlist>
811 </para>
812 </listitem>
813 </itemizedlist>
814 </para>
816 </sect2>
819 <sect2 id="drd-manual.lock-contention" xreflabel="Lock Contention">
820 <title>Detected Errors: Lock Contention</title>
822 <para>
823 Threads must be able to make progress without being blocked for too long by
824 other threads. Sometimes a thread has to wait until a mutex or reader-writer
825 synchronization object is unlocked by another thread. This is called
826 <emphasis>lock contention</emphasis>.
827 </para>
829 <para>
830 Lock contention causes delays. Such delays should be as short as
831 possible. The two command line options
832 <literal>--exclusive-threshold=<n></literal> and
833 <literal>--shared-threshold=<n></literal> make it possible to
834 detect excessive lock contention by making DRD report any lock that
835 has been held longer than the specified threshold. An example:
836 </para>
837 <programlisting><![CDATA[
838 $ valgrind --tool=drd --exclusive-threshold=10 drd/tests/hold_lock -i 500
839 ...
840 ==10668== Acquired at:
841 ==10668== at 0x4C267C8: pthread_mutex_lock (drd_pthread_intercepts.c:395)
842 ==10668== by 0x400D92: main (hold_lock.c:51)
843 ==10668== Lock on mutex 0x7fefffd50 was held during 503 ms (threshold: 10 ms).
844 ==10668== at 0x4C26ADA: pthread_mutex_unlock (drd_pthread_intercepts.c:441)
845 ==10668== by 0x400DB5: main (hold_lock.c:55)
846 ...
847 ]]></programlisting>
849 <para>
850 The <literal>hold_lock</literal> test program holds a lock as long as
851 specified by the <literal>-i</literal> (interval) argument. The DRD
852 output reports that the lock acquired at line 51 in source file
853 <literal>hold_lock.c</literal> and released at line 55 was held during
854 503 ms, while a threshold of 10 ms was specified to DRD.
855 </para>
857 </sect2>
860 <sect2 id="drd-manual.api-checks" xreflabel="API Checks">
861 <title>Detected Errors: Misuse of the POSIX threads API</title>
863 <para>
864 DRD is able to detect and report the following misuses of the POSIX
865 threads API:
866 <itemizedlist>
867 <listitem>
868 <para>
869 Passing the address of one type of synchronization object
870 (e.g. a mutex) to a POSIX API call that expects a pointer to
871 another type of synchronization object (e.g. a condition
872 variable).
873 </para>
874 </listitem>
875 <listitem>
876 <para>
877 Attempts to unlock a mutex that has not been locked.
878 </para>
879 </listitem>
880 <listitem>
881 <para>
882 Attempts to unlock a mutex that was locked by another thread.
883 </para>
884 </listitem>
885 <listitem>
886 <para>
887 Attempts to lock a mutex of type
888 <literal>PTHREAD_MUTEX_NORMAL</literal> or a spinlock
889 recursively.
890 </para>
891 </listitem>
892 <listitem>
893 <para>
894 Destruction or deallocation of a locked mutex.
895 </para>
896 </listitem>
897 <listitem>
898 <para>
899 Sending a signal to a condition variable while no lock is held
900 on the mutex associated with the condition variable.
901 </para>
902 </listitem>
903 <listitem>
904 <para>
905 Calling <function>pthread_cond_wait</function> on a mutex
906 that is not locked, that is locked by another thread or that
907 has been locked recursively.
908 </para>
909 </listitem>
910 <listitem>
911 <para>
912 Associating two different mutexes with a condition variable
913 through <function>pthread_cond_wait</function>.
914 </para>
915 </listitem>
916 <listitem>
917 <para>
918 Destruction or deallocation of a condition variable that is
919 being waited upon.
920 </para>
921 </listitem>
922 <listitem>
923 <para>
924 Destruction or deallocation of a locked reader-writer synchronization
925 object.
926 </para>
927 </listitem>
928 <listitem>
929 <para>
930 Attempts to unlock a reader-writer synchronization object that was not
931 locked by the calling thread.
932 </para>
933 </listitem>
934 <listitem>
935 <para>
936 Attempts to recursively lock a reader-writer synchronization object
937 exclusively.
938 </para>
939 </listitem>
940 <listitem>
941 <para>
942 Attempts to pass the address of a user-defined reader-writer
943 synchronization object to a POSIX threads function.
944 </para>
945 </listitem>
946 <listitem>
947 <para>
948 Attempts to pass the address of a POSIX reader-writer synchronization
949 object to one of the annotations for user-defined reader-writer
950 synchronization objects.
951 </para>
952 </listitem>
953 <listitem>
954 <para>
955 Reinitialization of a mutex, condition variable, reader-writer
956 lock, semaphore or barrier.
957 </para>
958 </listitem>
959 <listitem>
960 <para>
961 Destruction or deallocation of a semaphore or barrier that is
962 being waited upon.
963 </para>
964 </listitem>
965 <listitem>
966 <para>
967 Missing synchronization between barrier wait and barrier destruction.
968 </para>
969 </listitem>
970 <listitem>
971 <para>
972 Exiting a thread without first unlocking the spinlocks, mutexes or
973 reader-writer synchronization objects that were locked by that thread.
974 </para>
975 </listitem>
976 <listitem>
977 <para>
978 Passing an invalid thread ID to <function>pthread_join</function>
979 or <function>pthread_cancel</function>.
980 </para>
981 </listitem>
982 </itemizedlist>
983 </para>
985 </sect2>
988 <sect2 id="drd-manual.clientreqs" xreflabel="Client requests">
989 <title>Client Requests</title>
991 <para>
992 Just as for other Valgrind tools it is possible to let a client program
993 interact with the DRD tool through client requests. In addition to the
994 client requests several macros have been defined that allow to use the
995 client requests in a convenient way.
996 </para>
998 <para>
999 The interface between client programs and the DRD tool is defined in
1000 the header file <literal><valgrind/drd.h></literal>. The
1001 available macros and client requests are:
1002 <itemizedlist>
1003 <listitem>
1004 <para>
1005 The macro <literal>DRD_GET_VALGRIND_THREADID</literal> and the
1006 corresponding client
1007 request <varname>VG_USERREQ__DRD_GET_VALGRIND_THREAD_ID</varname>.
1008 Query the thread ID that has been assigned by the Valgrind core to the
1009 thread executing this client request. Valgrind's thread ID's start at
1010 one and are recycled in case a thread stops.
1011 </para>
1012 </listitem>
1013 <listitem>
1014 <para>
1015 The macro <literal>DRD_GET_DRD_THREADID</literal> and the corresponding
1016 client request <varname>VG_USERREQ__DRD_GET_DRD_THREAD_ID</varname>.
1017 Query the thread ID that has been assigned by DRD to the thread
1018 executing this client request. These are the thread ID's reported by DRD
1019 in data race reports and in trace messages. DRD's thread ID's start at
1020 one and are never recycled.
1021 </para>
1022 </listitem>
1023 <listitem>
1024 <para>
1025 The macros <literal>DRD_IGNORE_VAR(x)</literal>,
1026 <literal>ANNOTATE_TRACE_MEMORY(&x)</literal> and the corresponding
1027 client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>. Some
1028 applications contain intentional races. There exist e.g. applications
1029 where the same value is assigned to a shared variable from two different
1030 threads. It may be more convenient to suppress such races than to solve
1031 these. This client request allows one to suppress such races.
1032 </para>
1033 </listitem>
1034 <listitem>
1035 <para>
1036 The macro <literal>DRD_STOP_IGNORING_VAR(x)</literal> and the
1037 corresponding client request
1038 <varname>VG_USERREQ__DRD_FINISH_SUPPRESSION</varname>. Tell DRD
1039 to no longer ignore data races for the address range that was suppressed
1040 either via the macro <literal>DRD_IGNORE_VAR(x)</literal> or via the
1041 client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>.
1042 </para>
1043 </listitem>
1044 <listitem>
1045 <para>
1046 The macro <literal>DRD_TRACE_VAR(x)</literal>. Trace all load and store
1047 activity for the address range starting at <literal>&x</literal> and
1048 occupying <literal>sizeof(x)</literal> bytes. When DRD reports a data
1049 race on a specified variable, and it's not immediately clear which
1050 source code statements triggered the conflicting accesses, it can be
1051 very helpful to trace all activity on the offending memory location.
1052 </para>
1053 </listitem>
1054 <listitem>
1055 <para>
1056 The macro <literal>DRD_STOP_TRACING_VAR(x)</literal>. Stop tracing load
1057 and store activity for the address range starting
1058 at <literal>&x</literal> and occupying <literal>sizeof(x)</literal>
1059 bytes.
1060 </para>
1061 </listitem>
1062 <listitem>
1063 <para>
1064 The macro <literal>ANNOTATE_TRACE_MEMORY(&x)</literal>. Trace all
1065 load and store activity that touches at least the single byte at the
1066 address <literal>&x</literal>.
1067 </para>
1068 </listitem>
1069 <listitem>
1070 <para>
1071 The client request <varname>VG_USERREQ__DRD_START_TRACE_ADDR</varname>,
1072 which allows one to trace all load and store activity for the specified
1073 address range.
1074 </para>
1075 </listitem>
1076 <listitem>
1077 <para>
1078 The client
1079 request <varname>VG_USERREQ__DRD_STOP_TRACE_ADDR</varname>. Do no longer
1080 trace load and store activity for the specified address range.
1081 </para>
1082 </listitem>
1083 <listitem>
1084 <para>
1085 The macro <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> tells DRD to
1086 insert a mark. Insert this macro just after an access to the variable at
1087 the specified address has been performed.
1088 </para>
1089 </listitem>
1090 <listitem>
1091 <para>
1092 The macro <literal>ANNOTATE_HAPPENS_AFTER(addr)</literal> tells DRD that
1093 the next access to the variable at the specified address should be
1094 considered to have happened after the access just before the latest
1095 <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> annotation that
1096 references the same variable. The purpose of these two macros is to tell
1097 DRD about the order of inter-thread memory accesses implemented via
1098 atomic memory operations. See
1099 also <literal>drd/tests/annotate_smart_pointer.cpp</literal> for an
1100 example.
1101 </para>
1102 </listitem>
1103 <listitem>
1104 <para>
1105 The macro <literal>ANNOTATE_RWLOCK_CREATE(rwlock)</literal> tells DRD
1106 that the object at address <literal>rwlock</literal> is a
1107 reader-writer synchronization object that is not a
1108 <literal>pthread_rwlock_t</literal> synchronization object. See
1109 also <literal>drd/tests/annotate_rwlock.c</literal> for an example.
1110 </para>
1111 </listitem>
1112 <listitem>
1113 <para>
1114 The macro <literal>ANNOTATE_RWLOCK_DESTROY(rwlock)</literal> tells DRD
1115 that the reader-writer synchronization object at
1116 address <literal>rwlock</literal> has been destroyed.
1117 </para>
1118 </listitem>
1119 <listitem>
1120 <para>
1121 The macro <literal>ANNOTATE_WRITERLOCK_ACQUIRED(rwlock)</literal> tells
1122 DRD that a writer lock has been acquired on the reader-writer
1123 synchronization object at address <literal>rwlock</literal>.
1124 </para>
1125 </listitem>
1126 <listitem>
1127 <para>
1128 The macro <literal>ANNOTATE_READERLOCK_ACQUIRED(rwlock)</literal> tells
1129 DRD that a reader lock has been acquired on the reader-writer
1130 synchronization object at address <literal>rwlock</literal>.
1131 </para>
1132 </listitem>
1133 <listitem>
1134 <para>
1135 The macro <literal>ANNOTATE_RWLOCK_ACQUIRED(rwlock, is_w)</literal>
1136 tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that
1137 a reader lock (when <literal>is_w == 0</literal>) has been acquired on
1138 the reader-writer synchronization object at
1139 address <literal>rwlock</literal>.
1140 </para>
1141 </listitem>
1142 <listitem>
1143 <para>
1144 The macro <literal>ANNOTATE_WRITERLOCK_RELEASED(rwlock)</literal> tells
1145 DRD that a writer lock has been released on the reader-writer
1146 synchronization object at address <literal>rwlock</literal>.
1147 </para>
1148 </listitem>
1149 <listitem>
1150 <para>
1151 The macro <literal>ANNOTATE_READERLOCK_RELEASED(rwlock)</literal> tells
1152 DRD that a reader lock has been released on the reader-writer
1153 synchronization object at address <literal>rwlock</literal>.
1154 </para>
1155 </listitem>
1156 <listitem>
1157 <para>
1158 The macro <literal>ANNOTATE_RWLOCK_RELEASED(rwlock, is_w)</literal>
1159 tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that
1160 a reader lock (when <literal>is_w == 0</literal>) has been released on
1161 the reader-writer synchronization object at
1162 address <literal>rwlock</literal>.
1163 </para>
1164 </listitem>
1165 <listitem>
1166 <para>
1167 The macro <literal>ANNOTATE_BARRIER_INIT(barrier, count,
1168 reinitialization_allowed)</literal> tells DRD that a new barrier object
1169 at the address <literal>barrier</literal> has been initialized,
1170 that <literal>count</literal> threads participate in each barrier and
1171 also whether or not barrier reinitialization without intervening
1172 destruction should be reported as an error. See
1173 also <literal>drd/tests/annotate_barrier.c</literal> for an example.
1174 </para>
1175 </listitem>
1176 <listitem>
1177 <para>
1178 The macro <literal>ANNOTATE_BARRIER_DESTROY(barrier)</literal>
1179 tells DRD that a barrier object is about to be destroyed.
1180 </para>
1181 </listitem>
1182 <listitem>
1183 <para>
1184 The macro <literal>ANNOTATE_BARRIER_WAIT_BEFORE(barrier)</literal>
1185 tells DRD that waiting for a barrier will start.
1186 </para>
1187 </listitem>
1188 <listitem>
1189 <para>
1190 The macro <literal>ANNOTATE_BARRIER_WAIT_AFTER(barrier)</literal>
1191 tells DRD that waiting for a barrier has finished.
1192 </para>
1193 </listitem>
1194 <listitem>
1195 <para>
1196 The macro <literal>ANNOTATE_BENIGN_RACE_SIZED(addr, size,
1197 descr)</literal> tells DRD that any races detected on the specified
1198 address are benign and hence should not be
1199 reported. The <literal>descr</literal> argument is ignored but can be
1200 used to document why data races on <literal>addr</literal> are benign.
1201 </para>
1202 </listitem>
1203 <listitem>
1204 <para>
1205 The macro <literal>ANNOTATE_BENIGN_RACE_STATIC(var, descr)</literal>
1206 tells DRD that any races detected on the specified static variable are
1207 benign and hence should not be reported. The <literal>descr</literal>
1208 argument is ignored but can be used to document why data races
1209 on <literal>var</literal> are benign. Note: this macro can only be
1210 used in C++ programs and not in C programs.
1211 </para>
1212 </listitem>
1213 <listitem>
1214 <para>
1215 The macro <literal>ANNOTATE_IGNORE_READS_BEGIN</literal> tells
1216 DRD to ignore all memory loads performed by the current thread.
1217 </para>
1218 </listitem>
1219 <listitem>
1220 <para>
1221 The macro <literal>ANNOTATE_IGNORE_READS_END</literal> tells
1222 DRD to stop ignoring the memory loads performed by the current thread.
1223 </para>
1224 </listitem>
1225 <listitem>
1226 <para>
1227 The macro <literal>ANNOTATE_IGNORE_WRITES_BEGIN</literal> tells
1228 DRD to ignore all memory stores performed by the current thread.
1229 </para>
1230 </listitem>
1231 <listitem>
1232 <para>
1233 The macro <literal>ANNOTATE_IGNORE_WRITES_END</literal> tells
1234 DRD to stop ignoring the memory stores performed by the current thread.
1235 </para>
1236 </listitem>
1237 <listitem>
1238 <para>
1239 The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN</literal> tells
1240 DRD to ignore all memory accesses performed by the current thread.
1241 </para>
1242 </listitem>
1243 <listitem>
1244 <para>
1245 The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_END</literal> tells
1246 DRD to stop ignoring the memory accesses performed by the current thread.
1247 </para>
1248 </listitem>
1249 <listitem>
1250 <para>
1251 The macro <literal>ANNOTATE_NEW_MEMORY(addr, size)</literal> tells
1252 DRD that the specified memory range has been allocated by a custom
1253 memory allocator in the client program and that the client program
1254 will start using this memory range.
1255 </para>
1256 </listitem>
1257 <listitem>
1258 <para>
1259 The macro <literal>ANNOTATE_THREAD_NAME(name)</literal> tells DRD to
1260 associate the specified name with the current thread and to include this
1261 name in the error messages printed by DRD.
1262 </para>
1263 </listitem>
1264 <listitem>
1265 <para>
1266 The macros <literal>VALGRIND_MALLOCLIKE_BLOCK</literal> and
1267 <literal>VALGRIND_FREELIKE_BLOCK</literal> from the Valgrind core are
1268 implemented; they are described in
1269 <xref linkend="manual-core-adv.clientreq"/>.
1270 </para>
1271 </listitem>
1272 </itemizedlist>
1273 </para>
1275 <para>
1276 Note: if you compiled Valgrind yourself, the header file
1277 <literal><valgrind/drd.h></literal> will have been installed in
1278 the directory <literal>/usr/include</literal> by the command
1279 <literal>make install</literal>. If you obtained Valgrind by
1280 installing it as a package however, you will probably have to install
1281 another package with a name like <literal>valgrind-devel</literal>
1282 before Valgrind's header files are available.
1283 </para>
1285 </sect2>
1288 <sect2 id="drd-manual.CXX11" xreflabel="C++11">
1289 <title>Debugging C++11 Programs</title>
1291 <para>If you want to use the C++11 class std::thread you will need to do the
1292 following to annotate the std::shared_ptr<> objects used in the
1293 implementation of that class:
1294 <itemizedlist>
1295 <listitem>
1296 <para>Add the following code at the start of a common header or at the
1297 start of each source file, before any C++ header files are included:</para>
1298 <programlisting>
1299 #include <valgrind/drd.h>
1300 #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(addr) ANNOTATE_HAPPENS_BEFORE(addr)
1301 #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(addr) ANNOTATE_HAPPENS_AFTER(addr)
1302 </programlisting>
1303 </listitem>
1304 <listitem>
1305 <para>Download the gcc source code and from source file
1306 libstdc++-v3/src/c++11/thread.cc copy the implementation of the
1307 <computeroutput>execute_native_thread_routine()</computeroutput>
1308 and <computeroutput>std::thread::_M_start_thread()</computeroutput>
1309 functions into a source file that is linked with your application. Make
1310 sure that also in this source file the
1311 _GLIBCXX_SYNCHRONIZATION_HAPPENS_*() macros are defined properly.</para>
1312 </listitem>
1313 </itemizedlist>
1314 </para>
1315 <para>For more information, see also <emphasis>The
1316 GNU C++ Library Manual, Debugging Support</emphasis>
1317 (<ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html">http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html</ulink>).</para>
1319 </sect2>
1322 <sect2 id="drd-manual.gnome" xreflabel="GNOME">
1323 <title>Debugging GNOME Programs</title>
1325 <para>
1326 GNOME applications use the threading primitives provided by the
1327 <computeroutput>glib</computeroutput> and
1328 <computeroutput>gthread</computeroutput> libraries. These libraries
1329 are built on top of POSIX threads, and hence are directly supported by
1330 DRD. Please keep in mind that you have to call
1331 <function>g_thread_init</function> before creating any threads, or
1332 DRD will report several data races on glib functions. See also the
1333 <ulink
1334 url="http://library.gnome.org/devel/glib/stable/glib-Threads.html">GLib
1335 Reference Manual</ulink> for more information about
1336 <function>g_thread_init</function>.
1337 </para>
1339 <para>
1340 One of the many facilities provided by the <literal>glib</literal>
1341 library is a block allocator, called <literal>g_slice</literal>. You
1342 have to disable this block allocator when using DRD by adding the
1343 following to the shell environment variables:
1344 <literal>G_SLICE=always-malloc</literal>. See also the <ulink
1345 url="http://library.gnome.org/devel/glib/stable/glib-Memory-Slices.html">GLib
1346 Reference Manual</ulink> for more information.
1347 </para>
1349 </sect2>
1352 <sect2 id="drd-manual.boost.thread" xreflabel="Boost.Thread">
1353 <title>Debugging Boost.Thread Programs</title>
1355 <para>
1356 The Boost.Thread library is the threading library included with the
1357 cross-platform Boost Libraries. This threading library is an early
1358 implementation of the upcoming C++0x threading library.
1359 </para>
1361 <para>
1362 Applications that use the Boost.Thread library should run fine under DRD.
1363 </para>
1365 <para>
1366 More information about Boost.Thread can be found here:
1367 <itemizedlist>
1368 <listitem>
1369 <para>
1370 Anthony Williams, <ulink
1371 url="http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html">Boost.Thread</ulink>
1372 Library Documentation, Boost website, 2007.
1373 </para>
1374 </listitem>
1375 <listitem>
1376 <para>
1377 Anthony Williams, <ulink
1378 url="http://www.ddj.com/cpp/211600441">What's New in Boost
1379 Threads?</ulink>, Recent changes to the Boost Thread library,
1380 Dr. Dobbs Magazine, October 2008.
1381 </para>
1382 </listitem>
1383 </itemizedlist>
1384 </para>
1386 </sect2>
1389 <sect2 id="drd-manual.openmp" xreflabel="OpenMP">
1390 <title>Debugging OpenMP Programs</title>
1392 <para>
1393 OpenMP stands for <emphasis>Open Multi-Processing</emphasis>. The OpenMP
1394 standard consists of a set of compiler directives for C, C++ and Fortran
1395 programs that allows a compiler to transform a sequential program into a
1396 parallel program. OpenMP is well suited for HPC applications and allows one to
1397 work at a higher level compared to direct use of the POSIX threads API. While
1398 OpenMP ensures that the POSIX API is used correctly, OpenMP programs can still
1399 contain data races. So it definitely makes sense to verify OpenMP programs
1400 with a thread checking tool.
1401 </para>
1403 <para>
1404 DRD supports OpenMP shared-memory programs generated by GCC. GCC
1405 supports OpenMP since version 4.2.0. GCC's runtime support
1406 for OpenMP programs is provided by a library called
1407 <literal>libgomp</literal>. The synchronization primitives implemented
1408 in this library use Linux' futex system call directly, unless the
1409 library has been configured with the
1410 <literal>--disable-linux-futex</literal> option. DRD only supports
1411 libgomp libraries that have been configured with this option and in
1412 which symbol information is present. For most Linux distributions this
1413 means that you will have to recompile GCC. See also the script
1414 <literal>drd/scripts/download-and-build-gcc</literal> in the
1415 Valgrind source tree for an example of how to compile GCC. You will
1416 also have to make sure that the newly compiled
1417 <literal>libgomp.so</literal> library is loaded when OpenMP programs
1418 are started. This is possible by adding a line similar to the
1419 following to your shell startup script:
1420 </para>
1421 <programlisting><![CDATA[
1422 export LD_LIBRARY_PATH=~/gcc-4.4.0/lib64:~/gcc-4.4.0/lib:
1423 ]]></programlisting>
1425 <para>
1426 As an example, the test OpenMP test program
1427 <literal>drd/tests/omp_matinv</literal> triggers a data race
1428 when the option -r has been specified on the command line. The data
1429 race is triggered by the following code:
1430 </para>
1431 <programlisting><![CDATA[
1432 #pragma omp parallel for private(j)
1433 for (j = 0; j < rows; j++)
1434 {
1435 if (i != j)
1436 {
1437 const elem_t factor = a[j * cols + i];
1438 for (k = 0; k < cols; k++)
1439 {
1440 a[j * cols + k] -= a[i * cols + k] * factor;
1441 }
1442 }
1443 }
1444 ]]></programlisting>
1446 <para>
1447 The above code is racy because the variable <literal>k</literal> has
1448 not been declared private. DRD will print the following error message
1449 for the above code:
1450 </para>
1451 <programlisting><![CDATA[
1452 $ valgrind --tool=drd --check-stack-var=yes --read-var-info=yes drd/tests/omp_matinv 3 -t 2 -r
1453 ...
1454 Conflicting store by thread 1/1 at 0x7fefffbc4 size 4
1455 at 0x4014A0: gj.omp_fn.0 (omp_matinv.c:203)
1456 by 0x401211: gj (omp_matinv.c:159)
1457 by 0x40166A: invert_matrix (omp_matinv.c:238)
1458 by 0x4019B4: main (omp_matinv.c:316)
1459 Location 0x7fefffbc4 is 0 bytes inside local var "k"
1460 declared at omp_matinv.c:160, in frame #0 of thread 1
1461 ...
1462 ]]></programlisting>
1463 <para>
1464 In the above output the function name <function>gj.omp_fn.0</function>
1465 has been generated by GCC from the function name
1466 <function>gj</function>. The allocation context information shows that the
1467 data race has been caused by modifying the variable <literal>k</literal>.
1468 </para>
1470 <para>
1471 Note: for GCC versions before 4.4.0, no allocation context information is
1472 shown. With these GCC versions the most usable information in the above output
1473 is the source file name and the line number where the data race has been
1474 detected (<literal>omp_matinv.c:203</literal>).
1475 </para>
1477 <para>
1478 For more information about OpenMP, see also
1479 <ulink url="http://openmp.org/">openmp.org</ulink>.
1480 </para>
1482 </sect2>
1485 <sect2 id="drd-manual.cust-mem-alloc" xreflabel="Custom Memory Allocators">
1486 <title>DRD and Custom Memory Allocators</title>
1488 <para>
1489 DRD tracks all memory allocation events that happen via the
1490 standard memory allocation and deallocation functions
1491 (<function>malloc</function>, <function>free</function>,
1492 <function>new</function> and <function>delete</function>), via entry
1493 and exit of stack frames or that have been annotated with Valgrind's
1494 memory pool client requests. DRD uses memory allocation and deallocation
1495 information for two purposes:
1496 <itemizedlist>
1497 <listitem>
1498 <para>
1499 To know where the scope ends of POSIX objects that have not been
1500 destroyed explicitly. It is e.g. not required by the POSIX
1501 threads standard to call
1502 <function>pthread_mutex_destroy</function> before freeing the
1503 memory in which a mutex object resides.
1504 </para>
1505 </listitem>
1506 <listitem>
1507 <para>
1508 To know where the scope of variables ends. If e.g. heap memory
1509 has been used by one thread, that thread frees that memory, and
1510 another thread allocates and starts using that memory, no data
1511 races must be reported for that memory.
1512 </para>
1513 </listitem>
1514 </itemizedlist>
1515 </para>
1517 <para>
1518 It is essential for correct operation of DRD that the tool knows about
1519 memory allocation and deallocation events. When analyzing a client program
1520 with DRD that uses a custom memory allocator, either instrument the custom
1521 memory allocator with the <literal>VALGRIND_MALLOCLIKE_BLOCK</literal>
1522 and <literal>VALGRIND_FREELIKE_BLOCK</literal> macros or disable the
1523 custom memory allocator.
1524 </para>
1526 <para>
1527 As an example, the GNU libstdc++ library can be configured
1528 to use standard memory allocation functions instead of memory pools by
1529 setting the environment variable
1530 <literal>GLIBCXX_FORCE_NEW</literal>. For more information, see also
1531 the <ulink
1532 url="https://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html">libstdc++
1533 manual</ulink>.
1534 </para>
1536 </sect2>
1539 <sect2 id="drd-manual.drd-versus-memcheck" xreflabel="DRD Versus Memcheck">
1540 <title>DRD Versus Memcheck</title>
1542 <para>
1543 It is essential for correct operation of DRD that there are no memory
1544 errors such as dangling pointers in the client program. Which means that
1545 it is a good idea to make sure that your program is Memcheck-clean
1546 before you analyze it with DRD. It is possible however that some of
1547 the Memcheck reports are caused by data races. In this case it makes
1548 sense to run DRD before Memcheck.
1549 </para>
1551 <para>
1552 So which tool should be run first? In case both DRD and Memcheck
1553 complain about a program, a possible approach is to run both tools
1554 alternatingly and to fix as many errors as possible after each run of
1555 each tool until none of the two tools prints any more error messages.
1556 </para>
1558 </sect2>
1561 <sect2 id="drd-manual.resource-requirements" xreflabel="Resource Requirements">
1562 <title>Resource Requirements</title>
1564 <para>
1565 The requirements of DRD with regard to heap and stack memory and the
1566 effect on the execution time of client programs are as follows:
1567 <itemizedlist>
1568 <listitem>
1569 <para>
1570 When running a program under DRD with default DRD options,
1571 between 1.1 and 3.6 times more memory will be needed compared to
1572 a native run of the client program. More memory will be needed
1573 if loading debug information has been enabled
1574 (<literal>--read-var-info=yes</literal>).
1575 </para>
1576 </listitem>
1577 <listitem>
1578 <para>
1579 DRD allocates some of its temporary data structures on the stack
1580 of the client program threads. This amount of data is limited to
1581 1 - 2 KB. Make sure that thread stacks are sufficiently large.
1582 </para>
1583 </listitem>
1584 <listitem>
1585 <para>
1586 Most applications will run between 20 and 50 times slower under
1587 DRD than a native single-threaded run. The slowdown will be most
1588 noticeable for applications which perform frequent mutex lock /
1589 unlock operations.
1590 </para>
1591 </listitem>
1592 </itemizedlist>
1593 </para>
1595 </sect2>
1598 <sect2 id="drd-manual.effective-use" xreflabel="Effective Use">
1599 <title>Hints and Tips for Effective Use of DRD</title>
1601 <para>
1602 The following information may be helpful when using DRD:
1603 <itemizedlist>
1604 <listitem>
1605 <para>
1606 Make sure that debug information is present in the executable
1607 being analyzed, such that DRD can print function name and line
1608 number information in stack traces. Most compilers can be told
1609 to include debug information via compiler option
1610 <option>-g</option>.
1611 </para>
1612 </listitem>
1613 <listitem>
1614 <para>
1615 Compile with option <option>-O1</option> instead of
1616 <option>-O0</option>. This will reduce the amount of generated
1617 code, may reduce the amount of debug info and will speed up
1618 DRD's processing of the client program. For more information,
1619 see also <xref linkend="manual-core.started"/>.
1620 </para>
1621 </listitem>
1622 <listitem>
1623 <para>
1624 If DRD reports any errors on libraries that are part of your
1625 Linux distribution like e.g. <literal>libc.so</literal> or
1626 <literal>libstdc++.so</literal>, installing the debug packages
1627 for these libraries will make the output of DRD a lot more
1628 detailed.
1629 </para>
1630 </listitem>
1631 <listitem>
1632 <para>
1633 When using C++, do not send output from more than one thread to
1634 <literal>std::cout</literal>. Doing so would not only
1635 generate multiple data race reports, it could also result in
1636 output from several threads getting mixed up. Either use
1637 <function>printf</function> or do the following:
1638 <orderedlist>
1639 <listitem>
1640 <para>Derive a class from <literal>std::ostreambuf</literal>
1641 and let that class send output line by line to
1642 <literal>stdout</literal>. This will avoid that individual
1643 lines of text produced by different threads get mixed
1644 up.</para>
1645 </listitem>
1646 <listitem>
1647 <para>Create one instance of <literal>std::ostream</literal>
1648 for each thread. This makes stream formatting settings
1649 thread-local. Pass a per-thread instance of the class
1650 derived from <literal>std::ostreambuf</literal> to the
1651 constructor of each instance. </para>
1652 </listitem>
1653 <listitem>
1654 <para>Let each thread send its output to its own instance of
1655 <literal>std::ostream</literal> instead of
1656 <literal>std::cout</literal>.</para>
1657 </listitem>
1658 </orderedlist>
1659 </para>
1660 </listitem>
1661 </itemizedlist>
1662 </para>
1664 </sect2>
1667 </sect1>
1670 <sect1 id="drd-manual.Pthreads" xreflabel="Pthreads">
1671 <title>Using the POSIX Threads API Effectively</title>
1673 <sect2 id="drd-manual.mutex-types" xreflabel="mutex-types">
1674 <title>Mutex types</title>
1676 <para>
1677 The Single UNIX Specification version two defines the following four
1678 mutex types (see also the documentation of <ulink
1679 url="http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_settype.html"><function>pthread_mutexattr_settype</function></ulink>):
1680 <itemizedlist>
1681 <listitem>
1682 <para>
1683 <emphasis>normal</emphasis>, which means that no error checking
1684 is performed, and that the mutex is non-recursive.
1685 </para>
1686 </listitem>
1687 <listitem>
1688 <para>
1689 <emphasis>error checking</emphasis>, which means that the mutex
1690 is non-recursive and that error checking is performed.
1691 </para>
1692 </listitem>
1693 <listitem>
1694 <para>
1695 <emphasis>recursive</emphasis>, which means that a mutex may be
1696 locked recursively.
1697 </para>
1698 </listitem>
1699 <listitem>
1700 <para>
1701 <emphasis>default</emphasis>, which means that error checking
1702 behavior is undefined, and that the behavior for recursive
1703 locking is also undefined. Or: portable code must neither
1704 trigger error conditions through the Pthreads API nor attempt to
1705 lock a mutex of default type recursively.
1706 </para>
1707 </listitem>
1708 </itemizedlist>
1709 </para>
1711 <para>
1712 In complex applications it is not always clear from beforehand which
1713 mutex will be locked recursively and which mutex will not be locked
1714 recursively. Attempts lock a non-recursive mutex recursively will
1715 result in race conditions that are very hard to find without a thread
1716 checking tool. So either use the error checking mutex type and
1717 consistently check the return value of Pthread API mutex calls, or use
1718 the recursive mutex type.
1719 </para>
1721 </sect2>
1723 <sect2 id="drd-manual.condvar" xreflabel="condition-variables">
1724 <title>Condition variables</title>
1726 <para>
1727 A condition variable allows one thread to wake up one or more other
1728 threads. Condition variables are often used to notify one or more
1729 threads about state changes of shared data. Unfortunately it is very
1730 easy to introduce race conditions by using condition variables as the
1731 only means of state information propagation. A better approach is to
1732 let threads poll for changes of a state variable that is protected by
1733 a mutex, and to use condition variables only as a thread wakeup
1734 mechanism. See also the source file
1735 <computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an
1736 example of how to implement this concept in C++. The monitor concept
1737 used in this example is a well known and very useful concept -- see
1738 also Wikipedia for more information about the <ulink
1739 url="http://en.wikipedia.org/wiki/Monitor_(synchronization)">monitor</ulink>
1740 concept.
1741 </para>
1743 </sect2>
1745 <sect2 id="drd-manual.pctw" xreflabel="pthread_cond_timedwait">
1746 <title>pthread_cond_timedwait and timeouts</title>
1748 <para>
1749 Historically the function
1750 <function>pthread_cond_timedwait</function> only allowed the
1751 specification of an absolute timeout, that is a timeout independent of
1752 the time when this function was called. However, almost every call to
1753 this function expresses a relative timeout. This typically happens by
1754 passing the sum of
1755 <computeroutput>clock_gettime(CLOCK_REALTIME)</computeroutput> and a
1756 relative timeout as the third argument. This approach is incorrect
1757 since forward or backward clock adjustments by e.g. ntpd will affect
1758 the timeout. A more reliable approach is as follows:
1759 <itemizedlist>
1760 <listitem>
1761 <para>
1762 When initializing a condition variable through
1763 <function>pthread_cond_init</function>, specify that the timeout of
1764 <function>pthread_cond_timedwait</function> will use the clock
1765 <literal>CLOCK_MONOTONIC</literal> instead of
1766 <literal>CLOCK_REALTIME</literal>. You can do this via
1767 <computeroutput>pthread_condattr_setclock(...,
1768 CLOCK_MONOTONIC)</computeroutput>.
1769 </para>
1770 </listitem>
1771 <listitem>
1772 <para>
1773 When calling <function>pthread_cond_timedwait</function>, pass
1774 the sum of
1775 <computeroutput>clock_gettime(CLOCK_MONOTONIC)</computeroutput>
1776 and a relative timeout as the third argument.
1777 </para>
1778 </listitem>
1779 </itemizedlist>
1780 See also
1781 <computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an
1782 example.
1783 </para>
1785 </sect2>
1787 </sect1>
1790 <sect1 id="drd-manual.limitations" xreflabel="Limitations">
1791 <title>Limitations</title>
1793 <para>DRD currently has the following limitations:</para>
1795 <itemizedlist>
1796 <listitem>
1797 <para>
1798 DRD, just like Memcheck, will refuse to start on Linux
1799 distributions where all symbol information has been removed from
1800 <filename>ld.so</filename>. This is e.g. the case for the PPC editions
1801 of openSUSE and Gentoo. You will have to install the glibc debuginfo
1802 package on these platforms before you can use DRD. See also openSUSE
1803 bug <ulink url="http://bugzilla.novell.com/show_bug.cgi?id=396197">
1804 396197</ulink> and Gentoo bug <ulink
1805 url="http://bugs.gentoo.org/214065">214065</ulink>.
1806 </para>
1807 </listitem>
1808 <listitem>
1809 <para>
1810 With gcc 4.4.3 and before, DRD may report data races on the C++
1811 class <literal>std::string</literal> in a multithreaded program. This is
1812 a know <literal>libstdc++</literal> issue -- see also GCC bug
1813 <ulink url="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40518">40518</ulink>
1814 for more information.
1815 </para>
1816 </listitem>
1817 <listitem>
1818 <para>
1819 If you compile the DRD source code yourself, you need GCC 3.0 or
1820 later. GCC 2.95 is not supported.
1821 </para>
1822 </listitem>
1823 <listitem>
1824 <para>
1825 Of the two POSIX threads implementations for Linux, only the
1826 NPTL (Native POSIX Thread Library) is supported. The older
1827 LinuxThreads library is not supported.
1828 </para>
1829 </listitem>
1830 </itemizedlist>
1832 </sect1>
1835 <sect1 id="drd-manual.feedback" xreflabel="Feedback">
1836 <title>Feedback</title>
1838 <para>
1839 If you have any comments, suggestions, feedback or bug reports about
1840 DRD, feel free to either post a message on the Valgrind users mailing
1841 list or to file a bug report. See also <ulink
1842 url="&vg-url;">&vg-url;</ulink> for more information.
1843 </para>
1845 </sect1>
1848 </chapter>