diff options
Diffstat (limited to 'nptl/pthread_once.c')
| -rw-r--r-- | nptl/pthread_once.c | 109 |
1 files changed, 93 insertions, 16 deletions
diff --git a/nptl/pthread_once.c b/nptl/pthread_once.c index 664b048acb..10c01d6023 100644 --- a/nptl/pthread_once.c +++ b/nptl/pthread_once.c @@ -1,6 +1,6 @@ -/* Copyright (C) 2002-2014 Free Software Foundation, Inc. +/* Copyright (C) 2003-2014 Free Software Foundation, Inc. This file is part of the GNU C Library. - Contributed by Ulrich Drepper <drepper@redhat.com>, 2002. + 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 @@ -9,7 +9,7 @@ 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 + 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 @@ -18,37 +18,114 @@ #include "pthreadP.h" #include <lowlevellock.h> +#include <atomic.h> +unsigned long int __fork_generation attribute_hidden; -static int once_lock = LLL_LOCK_INITIALIZER; +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. */ + *once_control = 0; + lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE); +} + +/* This is similar to a lock implementation, but we distinguish between three + states: not yet initialized (0), initialization finished (2), and + initialization in progress (__fork_generation | 1). 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. */ int __pthread_once (once_control, init_routine) pthread_once_t *once_control; void (*init_routine) (void); { - /* XXX Depending on whether the LOCK_IN_ONCE_T is defined use a - global lock variable or one which is part of the pthread_once_t - object. */ - if (*once_control == PTHREAD_ONCE_INIT) + while (1) { - lll_lock (once_lock, LLL_PRIVATE); + int oldval, val, newval; - /* XXX This implementation is not complete. It doesn't take - cancelation and fork into account. */ - if (*once_control == PTHREAD_ONCE_INIT) + /* We need acquire memory order for this load because if the value + signals that initialization has finished, we need to be see any + data modifications done during initialization. */ + val = *once_control; + atomic_read_barrier(); + do { - init_routine (); + /* Check if the initialization has already been done. */ + if (__glibc_likely ((val & 2) != 0)) + return 0; + + oldval = val; + /* 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 | 1; + /* We need acquire memory order here for the same reason as for the + load from once_control above. */ + val = atomic_compare_and_exchange_val_acq (once_control, newval, + oldval); + } + while (__glibc_unlikely (val != oldval)); - *once_control = !PTHREAD_ONCE_INIT; + /* Check if another thread already runs the initializer. */ + if ((oldval & 1) != 0) + { + /* Check whether the initializer execution was interrupted by a + fork. We know that for both values, bit 0 is set and bit 1 is + not. */ + if (oldval == newval) + { + /* Same generation, some other thread was faster. Wait. */ + lll_futex_wait (once_control, newval, LLL_PRIVATE); + continue; + } } - lll_unlock (once_lock, LLL_PRIVATE); + /* 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_write_barrier(); + *once_control = 2; + + /* Wake up all other threads. */ + lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE); + break; } return 0; } -strong_alias (__pthread_once, pthread_once) +weak_alias (__pthread_once, pthread_once) hidden_def (__pthread_once) |