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/* Commit an elided pthread lock.
Copyright (C) 2014-2021 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
<https://www.gnu.org/licenses/>. */
#include <pthreadP.h>
#include <lowlevellock.h>
#include <htm.h>
int
__lll_unlock_elision(int *futex, short *adapt_count, int private)
{
/* If the lock is free, we elided the lock earlier. This does not
necessarily mean that we are in a transaction, because the user code may
have closed the transaction, but that is impossible to detect reliably.
Relaxed MO access to futex is sufficient because a correct program
will only release a lock it has acquired; therefore, it must either
changed the futex word's value to something !=0 or it must have used
elision; these are actions by the same thread, so these actions are
sequenced-before the relaxed load (and thus also happens-before the
relaxed load). Therefore, relaxed MO is sufficient. */
if (atomic_load_relaxed (futex) == 0)
{
__libc_tend ();
}
else
{
/* Update the adapt_count while unlocking before completing the critical
section. adapt_count is accessed concurrently outside of a
transaction or a critical section (e.g. in elision-lock.c). So we need
to use atomic accesses. However, the value of adapt_count is just a
hint, so relaxed MO accesses are sufficient.
If adapt_count would be decremented while locking, multiple
CPUs, trying to lock the acquired mutex, will decrement adapt_count to
zero and another CPU will try to start a transaction, which will be
immediately aborted as the mutex is locked.
The update of adapt_count is done before releasing the lock as POSIX'
mutex destruction requirements disallow accesses to the mutex after it
has been released and thus could have been acquired or destroyed by
another thread. */
short adapt_count_val = atomic_load_relaxed (adapt_count);
if (adapt_count_val > 0)
atomic_store_relaxed (adapt_count, adapt_count_val - 1);
lll_unlock ((*futex), private);
}
return 0;
}
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