1/* force-elision.h: Automatic enabling of elision for mutexes
2 Copyright (C) 2013-2020 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/* Automatically enable elision for existing user lock kinds. */
20#define FORCE_ELISION(m, s) \
21 if (__pthread_force_elision) \
22 { \
23 /* See concurrency notes regarding __kind in \
24 struct __pthread_mutex_s in \
25 sysdeps/nptl/bits/thread-shared-types.h. \
26 \
27 There are the following cases for the kind of a mutex \
28 (The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags \
29 PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where \
30 only one of both flags can be set): \
31 - both flags are not set: \
32 This is the first lock operation for this mutex. Enable \
33 elision as it is not enabled so far. \
34 Note: It can happen that multiple threads are calling e.g. \
35 pthread_mutex_lock at the same time as the first lock \
36 operation for this mutex. Then elision is enabled for this \
37 mutex by multiple threads. Storing with relaxed MO is enough \
38 as all threads will store the same new value for the kind of \
39 the mutex. But we have to ensure that we always use the \
40 elision path regardless if this thread has enabled elision or \
41 another one. \
42 \
43 - PTHREAD_MUTEX_ELISION_NP flag is set: \
44 Elision was already enabled for this mutex by a previous lock \
45 operation. See case above. Just use the elision path. \
46 \
47 - PTHREAD_MUTEX_NO_ELISION_NP flag is set: \
48 Elision was explicitly disabled by pthread_mutexattr_settype. \
49 Do not use the elision path. \
50 Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be \
51 changed after mutex initialization. */ \
52 int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind)); \
53 if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \
54 { \
55 mutex_kind |= PTHREAD_MUTEX_ELISION_NP; \
56 atomic_store_relaxed (&((m)->__data.__kind), mutex_kind); \
57 } \
58 if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0) \
59 { \
60 s; \
61 } \
62 }
63