| /* Elided pthread mutex trylock. |
| Copyright (C) 2014-2018 Free Software Foundation, Inc. |
| This file is part of the GNU C Library. |
| |
| The GNU C Library is free software; you can redistribute it and/or |
| modify it under the terms of the GNU Lesser General Public |
| License as published by the Free Software Foundation; either |
| version 2.1 of the License, or (at your option) any later version. |
| |
| The GNU C Library is distributed in the hope that it will be useful, |
| but WITHOUT ANY WARRANTY; without even the implied warranty of |
| MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| Lesser General Public License for more details. |
| |
| You should have received a copy of the GNU Lesser General Public |
| License along with the GNU C Library; if not, see |
| <http://www.gnu.org/licenses/>. */ |
| |
| #include <pthread.h> |
| #include <pthreadP.h> |
| #include <lowlevellock.h> |
| #include <htm.h> |
| #include <elision-conf.h> |
| |
| #define aconf __elision_aconf |
| |
| /* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is |
| the adaptation counter in the mutex. */ |
| |
| int |
| __lll_trylock_elision (int *futex, short *adapt_count) |
| { |
| /* Implement POSIX semantics by forbiding nesting elided trylocks. |
| Sorry. After the abort the code is re-executed |
| non transactional and if the lock was already locked |
| return an error. */ |
| if (__libc_tx_nesting_depth () > 0) |
| { |
| /* Note that this abort may terminate an outermost transaction that |
| was created outside glibc. |
| This persistently aborts the current transactions to force |
| them to use the default lock instead of retrying transactions |
| until their try_tbegin is zero. |
| */ |
| __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); |
| __builtin_unreachable (); |
| } |
| |
| /* adapt_count can be accessed concurrently; these accesses can be both |
| inside of transactions (if critical sections are nested and the outer |
| critical section uses lock elision) and outside of transactions. Thus, |
| we need to use atomic accesses to avoid data races. However, the |
| value of adapt_count is just a hint, so relaxed MO accesses are |
| sufficient. */ |
| if (atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0) |
| { |
| int status = __libc_tbegin ((void *) 0); |
| if (__glibc_likely (status == _HTM_TBEGIN_STARTED)) |
| { |
| /* Check the futex to make sure nobody has touched it in the |
| mean time. This forces the futex into the cache and makes |
| sure the transaction aborts if another thread acquires the lock |
| concurrently. */ |
| if (__glibc_likely (atomic_load_relaxed (futex) == 0)) |
| /* Lock was free. Return to user code in a transaction. */ |
| return 0; |
| |
| /* Lock was busy. Fall back to normal locking. |
| This can be the case if e.g. adapt_count was decremented to zero |
| by a former release and another thread has been waken up and |
| acquired it. |
| Since we are in a non-nested transaction there is no need to abort, |
| which is expensive. Simply end the started transaction. */ |
| __libc_tend (); |
| /* Note: Changing the adapt_count here might abort a transaction on a |
| different CPU, but that could happen anyway when the futex is |
| acquired, so there's no need to check the nesting depth here. |
| See above for why relaxed MO is sufficient. */ |
| if (aconf.skip_lock_busy > 0) |
| atomic_store_relaxed (adapt_count, aconf.skip_lock_busy); |
| } |
| else if (status != _HTM_TBEGIN_TRANSIENT) |
| { |
| /* A persistent abort (cc 1 or 3) indicates that a retry is |
| probably futile. Use the normal locking now and for the |
| next couple of calls. |
| Be careful to avoid writing to the lock. */ |
| if (aconf.skip_trylock_internal_abort > 0) |
| *adapt_count = aconf.skip_trylock_internal_abort; |
| } |
| /* Could do some retries here. */ |
| } |
| |
| /* Use normal locking as fallback path if the transaction does not |
| succeed. */ |
| return lll_trylock (*futex); |
| } |
