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62 lines
2.7 KiB
C
62 lines
2.7 KiB
C
/* Commit an elided pthread lock.
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Copyright (C) 2014-2018 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 because a correct program
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will only release a lock it has acquired; therefore, it must either
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changed the futex word's value to something !=0 or it must have used
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elision; these are actions by the same thread, so these actions are
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sequenced-before the relaxed load (and thus also happens-before the
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relaxed load). Therefore, relaxed MO is sufficient. */
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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 a critical section (e.g. in elision-lock.c). So we need
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to use atomic accesses. However, the value of adapt_count is just a
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hint, so 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 acquired 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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The update of adapt_count is done before releasing the lock as POSIX'
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mutex destruction requirements disallow accesses to the mutex after it
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has been released and thus could have been acquired or destroyed by
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another thread. */
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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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