1/* elision-lock.c: Elided pthread mutex lock.
2 Copyright (C) 2011-2021 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 <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
44int
45__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
46{
47 /* adapt_count can be accessed concurrently; these accesses can be both
48 inside of transactions (if critical sections are nested and the outer
49 critical section uses lock elision) and outside of transactions. Thus,
50 we need to use atomic accesses to avoid data races. However, the
51 value of adapt_count is just a hint, so relaxed MO accesses are
52 sufficient. */
53 if (atomic_load_relaxed (adapt_count) <= 0)
54 {
55 unsigned status;
56 int try_xbegin;
57
58 for (try_xbegin = aconf.retry_try_xbegin;
59 try_xbegin > 0;
60 try_xbegin--)
61 {
62 if ((status = _xbegin()) == _XBEGIN_STARTED)
63 {
64 if (*futex == 0)
65 return 0;
66
67 /* Lock was busy. Fall back to normal locking.
68 Could also _xend here but xabort with 0xff code
69 is more visible in the profiler. */
70 _xabort (_ABORT_LOCK_BUSY);
71 }
72
73 if (!(status & _XABORT_RETRY))
74 {
75 if ((status & _XABORT_EXPLICIT)
76 && _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
77 {
78 /* Right now we skip here. Better would be to wait a bit
79 and retry. This likely needs some spinning. See
80 above for why relaxed MO is sufficient. */
81 if (atomic_load_relaxed (adapt_count)
82 != aconf.skip_lock_busy)
83 atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
84 }
85 /* Internal abort. There is no chance for retry.
86 Use the normal locking and next time use lock.
87 Be careful to avoid writing to the lock. See above for why
88 relaxed MO is sufficient. */
89 else if (atomic_load_relaxed (adapt_count)
90 != aconf.skip_lock_internal_abort)
91 atomic_store_relaxed (adapt_count,
92 aconf.skip_lock_internal_abort);
93 break;
94 }
95 }
96 }
97 else
98 {
99 /* Use a normal lock until the threshold counter runs out.
100 Lost updates possible. */
101 atomic_store_relaxed (adapt_count,
102 atomic_load_relaxed (adapt_count) - 1);
103 }
104
105 /* Use a normal lock here. */
106 return LLL_LOCK ((*futex), private);
107}
108