nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c

   1 /* elision-lock.c: Elided pthread mutex lock.
   2    Copyright (C) 2011-2014 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 "hle.h"
  23 #include <elision-conf.h>
  24
  25 #if !defined(LLL_LOCK) && !defined(EXTRAARG)
  26 /* Make sure the configuration code is always linked in for static
  27    libraries.  */
  28 #include "elision-conf.c"
  29 #endif
  30
  31 #ifndef EXTRAARG
  32 #define EXTRAARG
  33 #endif
  34 #ifndef LLL_LOCK
  35 #define LLL_LOCK(a,b) lll_lock(a,b), 0
  36 #endif
  37
  38 #define aconf __elision_aconf
  39
  40 /* Adaptive lock using transactions.
  41    By default the lock region is run as a transaction, and when it
  42    aborts or the lock is busy the lock adapts itself.  */
  43
  44 int
  45 __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
  46 {
  47   if (*adapt_count <= 0)
  48     {
  49       unsigned status;
  50       int try_xbegin;
  51
  52       for (try_xbegin = aconf.retry_try_xbegin;
  53            try_xbegin > 0;
  54            try_xbegin--)
  55         {
  56           if ((status = _xbegin()) == _XBEGIN_STARTED)
  57             {
  58               if (*futex == 0)
  59                 return 0;
  60
  61               /* Lock was busy.  Fall back to normal locking.
  62                  Could also _xend here but xabort with 0xff code
  63                  is more visible in the profiler.  */
  64               _xabort (_ABORT_LOCK_BUSY);
  65             }
  66
  67           if (!(status & _XABORT_RETRY))
  68             {
  69               if ((status & _XABORT_EXPLICIT)
  70                         && _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
  71                 {
  72                   /* Right now we skip here.  Better would be to wait a bit
  73                      and retry.  This likely needs some spinning.  */
  74                   if (*adapt_count != aconf.skip_lock_busy)
  75                     *adapt_count = aconf.skip_lock_busy;
  76                 }
  77               /* Internal abort.  There is no chance for retry.
  78                  Use the normal locking and next time use lock.
  79                  Be careful to avoid writing to the lock.  */
  80               else if (*adapt_count != aconf.skip_lock_internal_abort)
  81                 *adapt_count = aconf.skip_lock_internal_abort;
  82               break;
  83             }
  84         }
  85     }
  86   else
  87     {
  88       /* Use a normal lock until the threshold counter runs out.
  89          Lost updates possible.  */
  90       (*adapt_count)--;
  91     }
  92
  93   /* Use a normal lock here.  */
  94   return LLL_LOCK ((*futex), private);
  95 }