sysdeps/unix/sysv/linux/s390/elision-lock.c

   1 /* Elided pthread mutex lock.
   2    Copyright (C) 2014-2017 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <pthread.h>
  20 #include <pthreadP.h>
  21 #include <lowlevellock.h>
  22 #include <htm.h>
  23 #include <elision-conf.h>
  24 #include <stdint.h>
  25
  26 #if !defined(LLL_LOCK) && !defined(EXTRAARG)
  27 /* Make sure the configuration code is always linked in for static
  28    libraries.  */
  29 #include "elision-conf.c"
  30 #endif
  31
  32 #ifndef EXTRAARG
  33 #define EXTRAARG
  34 #endif
  35 #ifndef LLL_LOCK
  36 #define LLL_LOCK(a,b) lll_lock(a,b), 0
  37 #endif
  38
  39 #define aconf __elision_aconf
  40
  41 /* Adaptive lock using transactions.
  42    By default the lock region is run as a transaction, and when it
  43    aborts or the lock is busy the lock adapts itself.  */
  44
  45 int
  46 __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
  47 {
  48   /* adapt_count can be accessed concurrently; these accesses can be both
  49      inside of transactions (if critical sections are nested and the outer
  50      critical section uses lock elision) and outside of transactions.  Thus,
  51      we need to use atomic accesses to avoid data races.  However, the
  52      value of adapt_count is just a hint, so relaxed MO accesses are
  53      sufficient.
  54      Do not begin a transaction if another cpu has locked the
  55      futex with normal locking.  If adapt_count is zero, it remains and the
  56      next pthread_mutex_lock call will try to start a transaction again.  */
  57     if (atomic_load_relaxed (futex) == 0
  58         && atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0)
  59     {
  60       int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1);
  61       if (__builtin_expect (status == _HTM_TBEGIN_STARTED,
  62                             _HTM_TBEGIN_STARTED))
  63         {
  64           /* Check the futex to make sure nobody has touched it in the
  65              mean time.  This forces the futex into the cache and makes
  66              sure the transaction aborts if some other cpu uses the
  67              lock (writes the futex).  */
  68           if (__builtin_expect (atomic_load_relaxed (futex) == 0, 1))
  69             /* Lock was free.  Return to user code in a transaction.  */
  70             return 0;
  71
  72           /* Lock was busy.  Fall back to normal locking.  */
  73           if (__builtin_expect (__libc_tx_nesting_depth () <= 1, 1))
  74             {
  75               /* In a non-nested transaction there is no need to abort,
  76                  which is expensive.  Simply end the started transaction.  */
  77               __libc_tend ();
  78               /* Don't try to use transactions for the next couple of times.
  79                  See above for why relaxed MO is sufficient.  */
  80               if (aconf.skip_lock_busy > 0)
  81                 atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
  82             }
  83           else /* nesting depth is > 1 */
  84             {
  85               /* A nested transaction will abort eventually because it
  86                  cannot make any progress before *futex changes back to 0.
  87                  So we may as well abort immediately.
  88                  This persistently aborts the outer transaction to force
  89                  the outer mutex use the default lock instead of retrying
  90                  with transactions until the try_tbegin of the outer mutex
  91                  is zero.
  92                  The adapt_count of this inner mutex is not changed,
  93                  because using the default lock with the inner mutex
  94                  would abort the outer transaction.  */
  95               __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
  96               __builtin_unreachable ();
  97             }
  98         }
  99       else if (status != _HTM_TBEGIN_TRANSIENT)
 100         {
 101           /* A persistent abort (cc 1 or 3) indicates that a retry is
 102              probably futile.  Use the normal locking now and for the
 103              next couple of calls.
 104              Be careful to avoid writing to the lock.  See above for why
 105              relaxed MO is sufficient.  */
 106           if (aconf.skip_lock_internal_abort > 0)
 107             atomic_store_relaxed (adapt_count,
 108                                   aconf.skip_lock_internal_abort);
 109         }
 110       else
 111         {
 112           /* The transaction failed for some retries with
 113              _HTM_TBEGIN_TRANSIENT.  Use the normal locking now and for the
 114              next couple of calls.  */
 115           if (aconf.skip_lock_out_of_tbegin_retries > 0)
 116             atomic_store_relaxed (adapt_count,
 117                                   aconf.skip_lock_out_of_tbegin_retries);
 118         }
 119     }
 120
 121   /* Use normal locking as fallback path if transaction does not succeed.  */
 122   return LLL_LOCK ((*futex), private);
 123 }