2 * Copyright (c) 2005 Jeffrey M. Hsu. All rights reserved.
4 * This code is derived from software contributed to The DragonFly Project
5 * by Jeffrey M. Hsu. and Matthew Dillon
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
15 * 3. Neither the name of The DragonFly Project nor the names of its
16 * contributors may be used to endorse or promote products derived
17 * from this software without specific, prior written permission.
19 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
22 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
23 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
24 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
25 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
26 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
27 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
28 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
29 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34 * The implementation is designed to avoid looping when compatible operations
37 * To acquire a spinlock we first increment lock. Then we check if lock
38 * meets our requirements. For an exclusive spinlock it must be 1, of a
39 * shared spinlock it must either be 1 or the SHARED_SPINLOCK bit must be set.
41 * Shared spinlock failure case: Decrement the count, loop until we can
42 * transition from 0 to SHARED_SPINLOCK|1, or until we find SHARED_SPINLOCK
43 * is set and increment the count.
45 * Exclusive spinlock failure case: While maintaining the count, clear the
46 * SHARED_SPINLOCK flag unconditionally. Then use an atomic add to transfer
47 * the count from the low bits to the high bits of lock. Then loop until
48 * all low bits are 0. Once the low bits drop to 0 we can transfer the
49 * count back with an atomic_cmpset_int(), atomically, and return.
51 #include <sys/param.h>
52 #include <sys/systm.h>
53 #include <sys/types.h>
54 #include <sys/kernel.h>
55 #include <sys/sysctl.h>
60 #include <machine/atomic.h>
61 #include <machine/cpu.h>
62 #include <machine/cpufunc.h>
63 #include <machine/specialreg.h>
64 #include <machine/clock.h>
65 #include <sys/indefinite2.h>
66 #include <sys/spinlock.h>
67 #include <sys/spinlock2.h>
70 struct spinlock pmap_spin
= SPINLOCK_INITIALIZER(pmap_spin
, "pmap_spin");
75 #if !defined(KTR_SPIN_CONTENTION)
76 #define KTR_SPIN_CONTENTION KTR_ALL
78 #define SPIN_STRING "spin=%p type=%c"
79 #define SPIN_ARG_SIZE (sizeof(void *) + sizeof(int))
81 KTR_INFO_MASTER(spin
);
83 KTR_INFO(KTR_SPIN_CONTENTION
, spin
, beg
, 0, SPIN_STRING
, SPIN_ARG_SIZE
);
84 KTR_INFO(KTR_SPIN_CONTENTION
, spin
, end
, 1, SPIN_STRING
, SPIN_ARG_SIZE
);
87 #define logspin(name, spin, type) \
88 KTR_LOG(spin_ ## name, spin, type)
91 static int spin_lock_test_mode
;
94 #ifdef DEBUG_LOCKS_LATENCY
96 __read_frequently
static long spinlocks_add_latency
;
97 SYSCTL_LONG(_debug
, OID_AUTO
, spinlocks_add_latency
, CTLFLAG_RW
,
98 &spinlocks_add_latency
, 0,
99 "Add spinlock latency");
103 __read_frequently
static long spin_backoff_max
= 4096;
104 SYSCTL_LONG(_debug
, OID_AUTO
, spin_backoff_max
, CTLFLAG_RW
,
105 &spin_backoff_max
, 0,
106 "Spinlock exponential backoff limit");
108 /* 1 << n clock cycles, approx */
109 __read_frequently
static long spin_window_shift
= 8;
110 SYSCTL_LONG(_debug
, OID_AUTO
, spin_window_shift
, CTLFLAG_RW
,
111 &spin_window_shift
, 0,
112 "Spinlock TSC windowing");
114 __read_frequently
int indefinite_uses_rdtsc
= 1;
115 SYSCTL_INT(_debug
, OID_AUTO
, indefinite_uses_rdtsc
, CTLFLAG_RW
,
116 &indefinite_uses_rdtsc
, 0,
117 "Indefinite code uses RDTSC");
120 * We contested due to another exclusive lock holder. We lose.
122 * We have to unwind the attempt and may acquire the spinlock
123 * anyway while doing so.
126 spin_trylock_contested(struct spinlock
*spin
)
128 globaldata_t gd
= mycpu
;
131 * Handle degenerate case, else fail.
133 if (atomic_cmpset_int(&spin
->lock
, SPINLOCK_SHARED
|0, 1))
135 /*atomic_add_int(&spin->lock, -1);*/
137 crit_exit_quick(gd
->gd_curthread
);
143 * The spin_lock() inline was unable to acquire the lock and calls this
144 * function with spin->lock already incremented, passing (spin->lock - 1)
145 * to the function (the result of the inline's fetchadd).
147 * Note that we implement both exclusive and shared spinlocks, so we cannot
148 * use atomic_swap_int(). Instead, we try to use atomic_fetchadd_int()
149 * to put most of the burden on the cpu. Atomic_cmpset_int() (cmpxchg)
150 * can cause a lot of unnecessary looping in situations where it is just
151 * trying to increment the count.
153 * Similarly, we leave the SHARED flag intact and incur slightly more
154 * overhead when switching from shared to exclusive. This allows us to
155 * use atomic_fetchadd_int() for both spinlock types in the critical
158 * The exponential (n^1.5) backoff algorithm is designed to both reduce
159 * cache bus contention between cpu cores and sockets, and to allow some
160 * bursting of exclusive locks in heavily contended situations to improve
163 * The exclusive lock priority mechanism prevents even heavily contended
164 * exclusive locks from being starved by shared locks
167 _spin_lock_contested(struct spinlock
*spin
, const char *ident
, int value
)
169 indefinite_info_t info
;
175 * WARNING! Caller has already incremented the lock. We must
176 * increment the count value (from the inline's fetch-add)
179 * Handle the degenerate case where the spinlock is flagged SHARED
180 * with only our reference. We can convert it to EXCLUSIVE.
182 if (value
== (SPINLOCK_SHARED
| 1) - 1) {
183 if (atomic_cmpset_int(&spin
->lock
, SPINLOCK_SHARED
| 1, 1))
186 /* ++value; value not used after this */
187 info
.type
= 0; /* avoid improper gcc warning */
188 info
.ident
= NULL
; /* avoid improper gcc warning */
189 info
.secs
= 0; /* avoid improper gcc warning */
190 info
.base
= 0; /* avoid improper gcc warning */
191 info
.count
= 0; /* avoid improper gcc warning */
195 * Transfer our exclusive request to the high bits and clear the
196 * SPINLOCK_SHARED bit if it was set. This makes the spinlock
197 * appear exclusive, preventing any NEW shared or exclusive
198 * spinlocks from being obtained while we wait for existing
199 * shared or exclusive holders to unlock.
201 * Don't tread on earlier exclusive waiters by stealing the lock
202 * away early if the low bits happen to now be 1.
204 * The shared unlock understands that this may occur.
206 ovalue
= atomic_fetchadd_int(&spin
->lock
, SPINLOCK_EXCLWAIT
- 1);
207 ovalue
+= SPINLOCK_EXCLWAIT
- 1;
208 if (ovalue
& SPINLOCK_SHARED
) {
209 atomic_clear_int(&spin
->lock
, SPINLOCK_SHARED
);
210 ovalue
&= ~SPINLOCK_SHARED
;
214 expbackoff
= (expbackoff
+ 1) * 3 / 2;
215 if (expbackoff
== 6) /* 1, 3, 6, 10, ... */
216 indefinite_init(&info
, ident
, 0, 'S');
217 if (indefinite_uses_rdtsc
) {
218 if ((rdtsc() >> spin_window_shift
) % ncpus
!= mycpuid
) {
219 for (loop
= expbackoff
; loop
; --loop
)
226 * If the low bits are zero, try to acquire the exclusive lock
227 * by transfering our high bit reservation to the low bits.
229 * NOTE: Avoid unconditional atomic op by testing ovalue,
230 * otherwise we get cache bus armageddon.
232 * NOTE: We must also ensure that the SHARED bit is cleared.
233 * It is possible for it to wind up being set on a
234 * shared lock override of the EXCLWAIT bits.
238 if ((ovalue
& (SPINLOCK_EXCLWAIT
- 1)) == 0) {
241 nvalue
= ((ovalue
- SPINLOCK_EXCLWAIT
) | 1) &
243 if (atomic_fcmpset_int(&spin
->lock
, &ovalue
, nvalue
))
247 if (expbackoff
> 6 + spin_backoff_max
)
248 expbackoff
= 6 + spin_backoff_max
;
249 if (expbackoff
>= 6) {
250 if (indefinite_check(&info
))
255 indefinite_done(&info
);
259 * The spin_lock_shared() inline was unable to acquire the lock and calls
260 * this function with spin->lock already incremented.
262 * This is not in the critical path unless there is contention between
263 * shared and exclusive holders.
265 * Exclusive locks have priority over shared locks. However, this can
266 * cause shared locks to be starved when large numbers of threads are
267 * competing for exclusive locks so the shared lock code uses TSC-windowing
268 * to selectively ignore the exclusive priority mechanism. This has the
269 * effect of allowing a limited number of shared locks to compete against
270 * exclusive waiters at any given moment.
272 * Note that shared locks do not implement exponential backoff. Instead,
273 * the shared lock simply polls the lock value. One cpu_pause() is built
274 * into indefinite_check().
277 _spin_lock_shared_contested(struct spinlock
*spin
, const char *ident
)
279 indefinite_info_t info
;
283 * Undo the inline's increment.
285 ovalue
= atomic_fetchadd_int(&spin
->lock
, -1) - 1;
287 indefinite_init(&info
, ident
, 0, 's');
290 #ifdef DEBUG_LOCKS_LATENCY
292 for (j
= spinlocks_add_latency
; j
> 0; --j
)
298 * Loop until we can acquire the shared spinlock. Note that
299 * the low bits can be zero while the high EXCLWAIT bits are
300 * non-zero. In this situation exclusive requesters have
301 * priority (otherwise shared users on multiple cpus can hog
304 * NOTE: Reading spin->lock prior to the swap is extremely
305 * important on multi-chip/many-core boxes. On 48-core
306 * this one change improves fully concurrent all-cores
307 * compiles by 100% or better.
309 * I can't emphasize enough how important the pre-read
310 * is in preventing hw cache bus armageddon on
311 * multi-chip systems. And on single-chip/multi-core
312 * systems it just doesn't hurt.
317 * Ignore the EXCLWAIT bits if we are inside our window.
319 * We must always use a windowing approach here or the
320 * EXCLWAIT bits can prevent the shared lock from ever
321 * resolving... permanent starvation.
323 * In addition, if we were to always ignore the EXCLWAIT
324 * bits overlapping shared locks can prevent an exclusive
325 * lock from ever resolving... permanent starvation again.
327 if (/*indefinite_uses_rdtsc &&*/
328 (ovalue
& (SPINLOCK_EXCLWAIT
- 1)) == 0 &&
329 (rdtsc() >> spin_window_shift
) % ncpus
== mycpuid
) {
330 if (atomic_fcmpset_int(&spin
->lock
, &ovalue
,
331 ovalue
| SPINLOCK_SHARED
| 1)) {
338 * Check ovalue tightly (no exponential backoff for shared
339 * locks, that would result in horrible performance. Instead,
340 * shared locks depend on the exclusive priority mechanism
341 * to avoid starving exclusive locks).
344 if (atomic_fcmpset_int(&spin
->lock
, &ovalue
,
345 SPINLOCK_SHARED
| 1)) {
352 * If SHARED is already set, go for the increment, improving
353 * the exclusive to multiple-readers transition.
355 if (ovalue
& SPINLOCK_SHARED
) {
356 ovalue
= atomic_fetchadd_int(&spin
->lock
, 1);
357 /* ovalue += 1; NOT NEEDED */
358 if (ovalue
& SPINLOCK_SHARED
)
360 ovalue
= atomic_fetchadd_int(&spin
->lock
, -1);
364 if (indefinite_check(&info
))
367 * ovalue was wrong anyway, just reload
371 indefinite_done(&info
);
375 * Automatically avoid use of rdtsc when running in a VM
378 spinlock_sysinit(void *dummy __unused
)
381 indefinite_uses_rdtsc
= 0;
383 SYSINIT(spinsysinit
, SI_BOOT2_PROC0
, SI_ORDER_FIRST
, spinlock_sysinit
, NULL
);
387 * If INVARIANTS is enabled various spinlock timing tests can be run
388 * by setting debug.spin_lock_test:
390 * 1 Test the indefinite wait code
391 * 2 Time the best-case exclusive lock overhead (spin_test_count)
392 * 3 Time the best-case shared lock overhead (spin_test_count)
397 static int spin_test_count
= 10000000;
398 SYSCTL_INT(_debug
, OID_AUTO
, spin_test_count
, CTLFLAG_RW
, &spin_test_count
, 0,
399 "Number of iterations to use for spinlock wait code test");
402 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS
)
404 struct spinlock spin
;
409 if ((error
= priv_check(curthread
, PRIV_ROOT
)) != 0)
411 if ((error
= SYSCTL_IN(req
, &value
, sizeof(value
))) != 0)
415 * Indefinite wait test
418 spin_init(&spin
, "sysctllock");
419 spin_lock(&spin
); /* force an indefinite wait */
420 spin_lock_test_mode
= 1;
422 spin_unlock(&spin
); /* Clean up the spinlock count */
424 spin_lock_test_mode
= 0;
428 * Time best-case exclusive spinlocks
431 globaldata_t gd
= mycpu
;
433 spin_init(&spin
, "sysctllocktest");
434 for (i
= spin_test_count
; i
> 0; --i
) {
435 _spin_lock_quick(gd
, &spin
, "test");
436 spin_unlock_quick(gd
, &spin
);
443 SYSCTL_PROC(_debug
, KERN_PROC_ALL
, spin_lock_test
, CTLFLAG_RW
|CTLTYPE_INT
,
444 0, 0, sysctl_spin_lock_test
, "I", "Test spinlock wait code");
446 #endif /* INVARIANTS */