1 /* elision-trylock.c: Lock eliding trylock for pthreads. 2 Copyright (C) 2013-2021 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 #define aconf __elision_aconf 26 27 /* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is 28 the adaptation counter in the mutex. */ 29 30 int __lll_trylock_elision(int * futex,short * adapt_count)31__lll_trylock_elision (int *futex, short *adapt_count) 32 { 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 36 return an error. */ 37 _xabort (_ABORT_NESTED_TRYLOCK); 38 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 41 just a hint. */ 42 if (atomic_load_relaxed (adapt_count) <= 0) 43 { 44 unsigned status; 45 46 if ((status = _xbegin()) == _XBEGIN_STARTED) 47 { 48 if (*futex == 0) 49 return 0; 50 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); 55 } 56 57 if (!(status & _XABORT_RETRY)) 58 { 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); 64 } 65 /* Could do some retries here. */ 66 } 67 else 68 { 69 /* Lost updates are possible but harmless (see above). */ 70 atomic_store_relaxed (adapt_count, 71 atomic_load_relaxed (adapt_count) - 1); 72 } 73 74 return lll_trylock (*futex); 75 } 76 libc_hidden_def (__lll_trylock_elision) 77