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

   1 /* Elided pthread mutex trylock.
   2    Copyright (C) 2014-2016 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
  25 #define aconf __elision_aconf
  26
  27 /* Try to elide a futex trylock.  FUTEX is the futex variable.  ADAPT_COUNT is
  28    the adaptation counter in the mutex.  */
  29
  30 int
  31 __lll_trylock_elision (int *futex, short *adapt_count)
  32 {
  33   /* Implement POSIX semantics by forbiding nesting elided trylocks.
  34      Sorry.  After the abort the code is re-executed
  35      non transactional and if the lock was already locked
  36      return an error.  */
  37   if (__libc_tx_nesting_depth () > 0)
  38     {
  39       /* Note that this abort may terminate an outermost transaction that
  40          was created outside glibc.
  41          This persistently aborts the current transactions to force
  42          them to use the default lock instead of retrying transactions
  43          until their try_tbegin is zero.
  44       */
  45       __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
  46     }
  47
  48   /* Only try a transaction if it's worth it.  See __lll_lock_elision for
  49      why we need atomic accesses.  Relaxed MO is sufficient because this is
  50      just a hint.  */
  51   if (atomic_load_relaxed (adapt_count) <= 0)
  52     {
  53       int status;
  54
  55       if (__builtin_expect
  56           ((status = __libc_tbegin ((void *) 0)) == _HTM_TBEGIN_STARTED, 1))
  57         {
  58           if (*futex == 0)
  59             return 0;
  60           /* Lock was busy.  Fall back to normal locking.  */
  61           /* Since we are in a non-nested transaction there is no need to abort,
  62              which is expensive.  */
  63           __libc_tend ();
  64           /* Note: Changing the adapt_count here might abort a transaction on a
  65              different cpu, but that could happen anyway when the futex is
  66              acquired, so there's no need to check the nesting depth here.
  67              See above for why relaxed MO is sufficient.  */
  68           if (aconf.skip_lock_busy > 0)
  69             atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
  70         }
  71       else
  72         {
  73           if (status != _HTM_TBEGIN_TRANSIENT)
  74             {
  75               /* A persistent abort (cc 1 or 3) indicates that a retry is
  76                  probably futile.  Use the normal locking now and for the
  77                  next couple of calls.
  78                  Be careful to avoid writing to the lock.  */
  79               if (aconf.skip_trylock_internal_abort > 0)
  80                 *adapt_count = aconf.skip_trylock_internal_abort;
  81             }
  82         }
  83       /* Could do some retries here.  */
  84     }
  85   else
  86     {
  87       /* Lost updates are possible, but harmless.  Due to races this might lead
  88          to *adapt_count becoming less than zero.  */
  89       atomic_store_relaxed (adapt_count,
  90                             atomic_load_relaxed (adapt_count) - 1);
  91     }
  92
  93   return lll_trylock (*futex);
  94 }