1 /* elision-lock.c: Elided pthread mutex lock.
2    Copyright (C) 2015-2021 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    <https://www.gnu.org/licenses/>.  */
18 
19 #include <stdio.h>
20 #include <pthread.h>
21 #include <pthreadP.h>
22 #include <lowlevellock.h>
23 #include <elision-conf.h>
24 #include "htm.h"
25 
26 #ifndef EXTRAARG
27 # define EXTRAARG
28 #endif
29 #ifndef LLL_LOCK
30 # define LLL_LOCK(a,b) lll_lock(a,b), 0
31 #endif
32 
33 #define aconf __elision_aconf
34 
35 /* Adaptive lock using transactions.
36    By default the lock region is run as a transaction, and when it
37    aborts or the lock is busy the lock adapts itself.  */
38 
39 int
__lll_lock_elision(int * lock,short * adapt_count,EXTRAARG int pshared)40 __lll_lock_elision (int *lock, short *adapt_count, EXTRAARG int pshared)
41 {
42   /* adapt_count is accessed concurrently but is just a hint.  Thus,
43      use atomic accesses but relaxed MO is sufficient.  */
44   if (atomic_load_relaxed (adapt_count) > 0)
45     {
46       goto use_lock;
47     }
48 
49   for (int i = aconf.try_tbegin; i > 0; i--)
50     {
51       if (__libc_tbegin (0))
52 	{
53 	  if (*lock == 0)
54 	    return 0;
55 	  /* Lock was busy.  Fall back to normal locking.  */
56 	  __libc_tabort (_ABORT_LOCK_BUSY);
57 	}
58       else
59 	{
60 	  /* A persistent failure indicates that a retry will probably
61 	     result in another failure.  Use normal locking now and
62 	     for the next couple of calls.  */
63 	  if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
64 	    {
65 	      if (aconf.skip_lock_internal_abort > 0)
66 		atomic_store_relaxed (adapt_count,
67 				      aconf.skip_lock_internal_abort);
68 	      goto use_lock;
69 	    }
70 	}
71      }
72 
73   /* Fall back to locks for a bit if retries have been exhausted */
74   if (aconf.try_tbegin > 0 && aconf.skip_lock_out_of_tbegin_retries > 0)
75     atomic_store_relaxed (adapt_count,
76 			  aconf.skip_lock_out_of_tbegin_retries);
77 
78 use_lock:
79   return LLL_LOCK ((*lock), pshared);
80 }
81 libc_hidden_def (__lll_lock_elision)
82