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/* Copyright (C) 2003-2021 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Jakub Jelinek <jakub@redhat.com>, 2003.
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 <futex-internal.h>
#include <atomic.h>
unsigned long int __fork_generation attribute_hidden;
static void
clear_once_control (void *arg)
{
pthread_once_t *once_control = (pthread_once_t *) arg;
/* Reset to the uninitialized state here. We don't need a stronger memory
order because we do not need to make any other of our writes visible to
other threads that see this value: This function will be called if we
get interrupted (see __pthread_once), so all we need to relay to other
threads is the state being reset again. */
atomic_store_relaxed (once_control, 0);
futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);
}
/* This is similar to a lock implementation, but we distinguish between three
states: not yet initialized (0), initialization in progress
(__fork_generation | __PTHREAD_ONCE_INPROGRESS), and initialization
finished (__PTHREAD_ONCE_DONE); __fork_generation does not use the bits
that are used for __PTHREAD_ONCE_INPROGRESS and __PTHREAD_ONCE_DONE (which
is what __PTHREAD_ONCE_FORK_GEN_INCR is used for). If in the first state,
threads will try to run the initialization by moving to the second state;
the first thread to do so via a CAS on once_control runs init_routine,
other threads block.
When forking the process, some threads can be interrupted during the second
state; they won't be present in the forked child, so we need to restart
initialization in the child. To distinguish an in-progress initialization
from an interrupted initialization (in which case we need to reclaim the
lock), we look at the fork generation that's part of the second state: We
can reclaim iff it differs from the current fork generation.
XXX: This algorithm has an ABA issue on the fork generation: If an
initialization is interrupted, we then fork 2^30 times (30 bits of
once_control are used for the fork generation), and try to initialize
again, we can deadlock because we can't distinguish the in-progress and
interrupted cases anymore.
XXX: We split out this slow path because current compilers do not generate
as efficient code when the fast path in __pthread_once below is not in a
separate function. */
static int
__attribute__ ((noinline))
__pthread_once_slow (pthread_once_t *once_control, void (*init_routine) (void))
{
while (1)
{
int val, newval;
/* We need acquire memory order for this load because if the value
signals that initialization has finished, we need to see any
data modifications done during initialization. */
val = atomic_load_acquire (once_control);
do
{
/* Check if the initialization has already been done. */
if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
return 0;
/* We try to set the state to in-progress and having the current
fork generation. We don't need atomic accesses for the fork
generation because it's immutable in a particular process, and
forked child processes start with a single thread that modified
the generation. */
newval = __fork_generation | __PTHREAD_ONCE_INPROGRESS;
/* We need acquire memory order here for the same reason as for the
load from once_control above. */
}
while (__glibc_unlikely (!atomic_compare_exchange_weak_acquire (
once_control, &val, newval)));
/* Check if another thread already runs the initializer. */
if ((val & __PTHREAD_ONCE_INPROGRESS) != 0)
{
/* Check whether the initializer execution was interrupted by a
fork. We know that for both values, __PTHREAD_ONCE_INPROGRESS
is set and __PTHREAD_ONCE_DONE is not. */
if (val == newval)
{
/* Same generation, some other thread was faster. Wait and
retry. */
futex_wait_simple ((unsigned int *) once_control,
(unsigned int) newval, FUTEX_PRIVATE);
continue;
}
}
/* This thread is the first here. Do the initialization.
Register a cleanup handler so that in case the thread gets
interrupted the initialization can be restarted. */
pthread_cleanup_push (clear_once_control, once_control);
init_routine ();
pthread_cleanup_pop (0);
/* Mark *once_control as having finished the initialization. We need
release memory order here because we need to synchronize with other
threads that want to use the initialized data. */
atomic_store_release (once_control, __PTHREAD_ONCE_DONE);
/* Wake up all other threads. */
futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);
break;
}
return 0;
}
int
__pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
{
/* Fast path. See __pthread_once_slow. */
int val;
val = atomic_load_acquire (once_control);
if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
return 0;
else
return __pthread_once_slow (once_control, init_routine);
}
weak_alias (__pthread_once, pthread_once)
hidden_def (__pthread_once)
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