1 /* elision-trylock.c: Lock eliding trylock for pthreads.
2 Copyright (C) 2013-2024 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
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.
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.
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/>. */
21 #include <lowlevellock.h>
23 #include <elision-conf.h>
25 #define aconf __elision_aconf
27 /* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is
28 the adaptation counter in the mutex. */
31 __lll_trylock_elision (int *futex
, short *adapt_count
)
33 /* Implement POSIX semantics by forbiding nesting
34 trylock. Sorry. After the abort the code is re-executed
35 non transactional and if the lock was already locked
37 _xabort (_ABORT_NESTED_TRYLOCK
);
39 /* Only try a transaction if it's worth it. See __lll_lock_elision for
40 why we need atomic accesses. Relaxed MO is sufficient because this is
42 if (atomic_load_relaxed (adapt_count
) <= 0)
46 if ((status
= _xbegin()) == _XBEGIN_STARTED
)
51 /* Lock was busy. Fall back to normal locking.
52 Could also _xend here but xabort with 0xff code
53 is more visible in the profiler. */
54 _xabort (_ABORT_LOCK_BUSY
);
57 if (!(status
& _XABORT_RETRY
))
59 /* Internal abort. No chance for retry. For future
60 locks don't try speculation for some time. See above for MO. */
61 if (atomic_load_relaxed (adapt_count
)
62 != aconf
.skip_lock_internal_abort
)
63 atomic_store_relaxed (adapt_count
, aconf
.skip_lock_internal_abort
);
65 /* Could do some retries here. */
69 /* Lost updates are possible but harmless (see above). */
70 atomic_store_relaxed (adapt_count
,
71 atomic_load_relaxed (adapt_count
) - 1);
74 return lll_trylock (*futex
);
76 libc_hidden_def (__lll_trylock_elision
)