| /* Elided pthread mutex lock. |
| 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> |
| #include <stdint.h> |
| |
| #if !defined(LLL_LOCK) && !defined(EXTRAARG) |
| /* Make sure the configuration code is always linked in for static |
| libraries. */ |
| #include "elision-conf.c" |
| #endif |
| |
| #ifndef EXTRAARG |
| #define EXTRAARG |
| #endif |
| #ifndef LLL_LOCK |
| #define LLL_LOCK(a,b) lll_lock(a,b), 0 |
| #endif |
| |
| #define aconf __elision_aconf |
| |
| /* Adaptive lock using transactions. |
| By default the lock region is run as a transaction, and when it |
| aborts or the lock is busy the lock adapts itself. */ |
| |
| int |
| __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) |
| { |
| /* 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) |
| { |
| /* Start a transaction and retry it automatically if it aborts with |
| _HTM_TBEGIN_TRANSIENT. This macro calls tbegin at most retry_cnt |
| + 1 times. The second argument is considered as retry_cnt. */ |
| int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1); |
| 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. */ |
| if (__glibc_likely (__libc_tx_nesting_depth () <= 1)) |
| { |
| /* In a non-nested transaction there is no need to abort, |
| which is expensive. Simply end the started transaction. */ |
| __libc_tend (); |
| /* Don't try to use transactions for the next couple of times. |
| See above for why relaxed MO is sufficient. */ |
| if (aconf.skip_lock_busy > 0) |
| atomic_store_relaxed (adapt_count, aconf.skip_lock_busy); |
| } |
| else /* nesting depth is > 1 */ |
| { |
| /* A nested transaction will abort eventually because it |
| cannot make any progress before *futex changes back to 0. |
| So we may as well abort immediately. |
| This persistently aborts the outer transaction to force |
| the outer mutex use the default lock instead of retrying |
| with transactions until the try_tbegin of the outer mutex |
| is zero. |
| The adapt_count of this inner mutex is not changed, |
| because using the default lock with the inner mutex |
| would abort the outer transaction. */ |
| __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); |
| __builtin_unreachable (); |
| } |
| } |
| 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. See above for why |
| relaxed MO is sufficient. */ |
| if (aconf.skip_lock_internal_abort > 0) |
| atomic_store_relaxed (adapt_count, |
| aconf.skip_lock_internal_abort); |
| } |
| else |
| { |
| /* The transaction failed for some retries with |
| _HTM_TBEGIN_TRANSIENT. Use the normal locking now and for the |
| next couple of calls. */ |
| if (aconf.skip_lock_out_of_tbegin_retries > 0) |
| atomic_store_relaxed (adapt_count, |
| aconf.skip_lock_out_of_tbegin_retries); |
| } |
| } |
| |
| /* Use normal locking as fallback path if the transaction does not |
| succeed. */ |
| return LLL_LOCK ((*futex), private); |
| } |