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The elision interfaces are closely aligned between the targets that implement them, so declare them in the generic <lowlevellock.h> file. Empty .c stubs are provided, so that fewer makefile updates under sysdeps are needed. Also simplify initialization via __libc_early_init. The symbols __lll_clocklock_elision, __lll_lock_elision, __lll_trylock_elision, __lll_unlock_elision, __pthread_force_elision move into libc. For the time being, non-hidden references are used from libpthread to access them, but once that part of libpthread is moved into libc, hidden symbols will be used again. (Hidden references seem desirable to reduce the likelihood of transactions aborts.)
63 lines
2.7 KiB
C
63 lines
2.7 KiB
C
/* Commit an elided pthread lock.
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Copyright (C) 2014-2021 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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<https://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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libc_hidden_def (__lll_unlock_elision)
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