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a2f0363f81
This adds new functions for futex operations, starting with wait, abstimed_wait, reltimed_wait, wake. They add documentation and error checking according to the current draft of the Linux kernel futex manpage. Waiting with absolute or relative timeouts is split into separate functions. This allows for removing a few cases of code duplication in pthreads code, which uses absolute timeouts; also, it allows us to put platform-specific code to go from an absolute to a relative timeout into the platform-specific futex abstractions.. Futex operations that can be canceled are also split out into separate functions suffixed by "_cancelable". There are separate versions for both Linux and NaCl; while they currently differ only slightly, my expectation is that the separate versions of lowlevellock-futex.h will eventually be merged into futex-internal.h when we get to move the lll_ functions over to the new futex API.
104 lines
3.2 KiB
C
104 lines
3.2 KiB
C
/* sem_post -- post to a POSIX semaphore. Generic futex-using version.
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Copyright (C) 2003-2015 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Jakub Jelinek <jakub@redhat.com>, 2003.
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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 <atomic.h>
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#include <errno.h>
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#include <sysdep.h>
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#include <lowlevellock.h> /* lll_futex* used by the old code. */
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#include <futex-internal.h>
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#include <internaltypes.h>
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#include <semaphore.h>
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#include <shlib-compat.h>
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/* See sem_wait for an explanation of the algorithm. */
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int
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__new_sem_post (sem_t *sem)
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{
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struct new_sem *isem = (struct new_sem *) sem;
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int private = isem->private;
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#if __HAVE_64B_ATOMICS
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/* Add a token to the semaphore. We use release MO to make sure that a
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thread acquiring this token synchronizes with us and other threads that
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added tokens before (the release sequence includes atomic RMW operations
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by other threads). */
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/* TODO Use atomic_fetch_add to make it scale better than a CAS loop? */
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uint64_t d = atomic_load_relaxed (&isem->data);
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do
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{
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if ((d & SEM_VALUE_MASK) == SEM_VALUE_MAX)
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{
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__set_errno (EOVERFLOW);
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return -1;
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}
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}
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while (!atomic_compare_exchange_weak_release (&isem->data, &d, d + 1));
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/* If there is any potentially blocked waiter, wake one of them. */
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if ((d >> SEM_NWAITERS_SHIFT) > 0)
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futex_wake (((unsigned int *) &isem->data) + SEM_VALUE_OFFSET, 1, private);
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#else
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/* Add a token to the semaphore. Similar to 64b version. */
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unsigned int v = atomic_load_relaxed (&isem->value);
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do
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{
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if ((v >> SEM_VALUE_SHIFT) == SEM_VALUE_MAX)
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{
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__set_errno (EOVERFLOW);
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return -1;
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}
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}
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while (!atomic_compare_exchange_weak_release
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(&isem->value, &v, v + (1 << SEM_VALUE_SHIFT)));
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/* If there is any potentially blocked waiter, wake one of them. */
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if ((v & SEM_NWAITERS_MASK) != 0)
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futex_wake (&isem->value, 1, private);
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#endif
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return 0;
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}
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versioned_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_1);
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#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
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int
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attribute_compat_text_section
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__old_sem_post (sem_t *sem)
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{
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int *futex = (int *) sem;
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/* We must need to synchronize with consumers of this token, so the atomic
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increment must have release MO semantics. */
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atomic_write_barrier ();
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(void) atomic_increment_val (futex);
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/* We always have to assume it is a shared semaphore. */
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int err = lll_futex_wake (futex, 1, LLL_SHARED);
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if (__builtin_expect (err, 0) < 0)
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{
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__set_errno (-err);
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return -1;
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}
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return 0;
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}
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compat_symbol (libpthread, __old_sem_post, sem_post, GLIBC_2_0);
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#endif
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