| 1 | /* elision-trylock.c: Lock eliding trylock for pthreads. |
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| 2 | Copyright (C) 2013-2022 Free Software Foundation, Inc. |
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| 3 | This file is part of the GNU C Library. |
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| 4 | |
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| 5 | The GNU C Library is free software; you can redistribute it and/or |
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| 6 | modify it under the terms of the GNU Lesser General Public |
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| 7 | License as published by the Free Software Foundation; either |
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| 8 | version 2.1 of the License, or (at your option) any later version. |
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| 9 | |
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| 10 | The GNU C Library is distributed in the hope that it will be useful, |
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| 11 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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| 12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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| 13 | Lesser General Public License for more details. |
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| 14 | |
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| 15 | You should have received a copy of the GNU Lesser General Public |
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| 16 | License along with the GNU C Library; if not, see |
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| 17 | <https://www.gnu.org/licenses/>. */ |
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| 18 | |
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| 19 | #include <pthread.h> |
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| 20 | #include <pthreadP.h> |
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| 21 | #include <lowlevellock.h> |
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| 22 | #include "hle.h" |
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| 23 | #include <elision-conf.h> |
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| 24 | |
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| 25 | #define aconf __elision_aconf |
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| 26 | |
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| 27 | /* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is |
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| 28 | the adaptation counter in the mutex. */ |
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| 29 | |
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| 30 | int |
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| 31 | __lll_trylock_elision (int *futex, short *adapt_count) |
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| 32 | { |
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| 33 | /* Implement POSIX semantics by forbiding nesting |
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| 34 | trylock. Sorry. After the abort the code is re-executed |
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| 35 | non transactional and if the lock was already locked |
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| 36 | return an error. */ |
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| 37 | _xabort (_ABORT_NESTED_TRYLOCK); |
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| 38 | |
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| 39 | /* Only try a transaction if it's worth it. See __lll_lock_elision for |
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| 40 | why we need atomic accesses. Relaxed MO is sufficient because this is |
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| 41 | just a hint. */ |
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| 42 | if (atomic_load_relaxed (adapt_count) <= 0) |
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| 43 | { |
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| 44 | unsigned status; |
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| 45 | |
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| 46 | if ((status = _xbegin()) == _XBEGIN_STARTED) |
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| 47 | { |
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| 48 | if (*futex == 0) |
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| 49 | return 0; |
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| 50 | |
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| 51 | /* Lock was busy. Fall back to normal locking. |
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| 52 | Could also _xend here but xabort with 0xff code |
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| 53 | is more visible in the profiler. */ |
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| 54 | _xabort (_ABORT_LOCK_BUSY); |
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| 55 | } |
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| 56 | |
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| 57 | if (!(status & _XABORT_RETRY)) |
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| 58 | { |
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| 59 | /* Internal abort. No chance for retry. For future |
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| 60 | locks don't try speculation for some time. See above for MO. */ |
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| 61 | if (atomic_load_relaxed (adapt_count) |
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| 62 | != aconf.skip_lock_internal_abort) |
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| 63 | atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort); |
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| 64 | } |
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| 65 | /* Could do some retries here. */ |
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| 66 | } |
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| 67 | else |
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| 68 | { |
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| 69 | /* Lost updates are possible but harmless (see above). */ |
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| 70 | atomic_store_relaxed (adapt_count, |
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| 71 | atomic_load_relaxed (adapt_count) - 1); |
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| 72 | } |
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| 73 | |
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| 74 | return lll_trylock (*futex); |
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| 75 | } |
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| 76 | libc_hidden_def (__lll_trylock_elision) |
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| 77 | |
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