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58 lines
2.4 KiB
C
58 lines
2.4 KiB
C
/* Commit an elided pthread lock.
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Copyright (C) 2014-2017 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <pthreadP.h>
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#include <lowlevellock.h>
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#include <htm.h>
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int
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__lll_unlock_elision(int *futex, short *adapt_count, int private)
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{
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/* If the lock is free, we elided the lock earlier. This does not
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necessarily mean that we are in a transaction, because the user code may
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have closed the transaction, but that is impossible to detect reliably.
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Relaxed MO access to futex is sufficient as we only need a hint, if we
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started a transaction or acquired the futex in e.g. elision-lock.c. */
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if (atomic_load_relaxed (futex) == 0)
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{
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__libc_tend ();
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}
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else
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{
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/* Update the adapt_count while unlocking before completing the critical
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section. adapt_count is accessed concurrently outside of a
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transaction or an aquired lock e.g. in elision-lock.c so we need to use
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atomic accesses. However, the value of adapt_count is just a hint, so
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relaxed MO accesses are sufficient.
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If adapt_count would be decremented while locking, multiple
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CPUs trying to lock the locked mutex will decrement adapt_count to
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zero and another CPU will try to start a transaction, which will be
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immediately aborted as the mutex is locked.
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If adapt_count would be decremented while unlocking after completing
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the critical section, possible waiters will be waked up before
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decrementing the adapt_count. Those waked up waiters could have
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destroyed and freed this mutex! */
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short adapt_count_val = atomic_load_relaxed (adapt_count);
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if (adapt_count_val > 0)
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atomic_store_relaxed (adapt_count, adapt_count_val - 1);
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lll_unlock ((*futex), private);
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}
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return 0;
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}
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