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/* $NetBSD: pthread_atfork.c,v 1.10 2015/01/20 18:31:25 christos Exp $ */
/*-
* Copyright (c) 2002 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This code is derived from software contributed to The NetBSD Foundation
* by Nathan J. Williams.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
#include <sys/cdefs.h>
#if defined(LIBC_SCCS) && !defined(lint)
__RCSID("$NetBSD: pthread_atfork.c,v 1.10 2015/01/20 18:31:25 christos Exp $");
#endif /* LIBC_SCCS and not lint */
#include "namespace.h"
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/queue.h>
#include "reentrant.h"
#ifdef __weak_alias
__weak_alias(pthread_atfork, _pthread_atfork)
__weak_alias(fork, _fork)
#endif /* __weak_alias */
pid_t __fork(void); /* XXX */
struct atfork_callback {
SIMPLEQ_ENTRY(atfork_callback) next;
void (*fn)(void);
};
/*
* Hypothetically, we could protect the queues with a rwlock which is
* write-locked by pthread_atfork() and read-locked by fork(), but
* since the intended use of the functions is obtaining locks to hold
* across the fork, forking is going to be serialized anyway.
*/
static struct atfork_callback atfork_builtin;
#ifdef _REENTRANT
static mutex_t atfork_lock = MUTEX_INITIALIZER;
#endif
SIMPLEQ_HEAD(atfork_callback_q, atfork_callback);
static struct atfork_callback_q prepareq = SIMPLEQ_HEAD_INITIALIZER(prepareq);
static struct atfork_callback_q parentq = SIMPLEQ_HEAD_INITIALIZER(parentq);
static struct atfork_callback_q childq = SIMPLEQ_HEAD_INITIALIZER(childq);
static struct atfork_callback *
af_alloc(void)
{
if (atfork_builtin.fn == NULL)
return &atfork_builtin;
return malloc(sizeof(atfork_builtin));
}
static void
af_free(struct atfork_callback *af)
{
if (af != &atfork_builtin)
free(af);
}
int
pthread_atfork(void (*prepare)(void), void (*parent)(void),
void (*child)(void))
{
struct atfork_callback *newprepare, *newparent, *newchild;
newprepare = newparent = newchild = NULL;
mutex_lock(&atfork_lock);
if (prepare != NULL) {
newprepare = af_alloc();
if (newprepare == NULL) {
mutex_unlock(&atfork_lock);
return ENOMEM;
}
newprepare->fn = prepare;
}
if (parent != NULL) {
newparent = af_alloc();
if (newparent == NULL) {
if (newprepare != NULL)
af_free(newprepare);
mutex_unlock(&atfork_lock);
return ENOMEM;
}
newparent->fn = parent;
}
if (child != NULL) {
newchild = af_alloc();
if (newchild == NULL) {
if (newprepare != NULL)
af_free(newprepare);
if (newparent != NULL)
af_free(newparent);
mutex_unlock(&atfork_lock);
return ENOMEM;
}
newchild->fn = child;
}
/*
* The order in which the functions are called is specified as
* LIFO for the prepare handler and FIFO for the others; insert
* at the head and tail as appropriate so that SIMPLEQ_FOREACH()
* produces the right order.
*/
if (prepare)
SIMPLEQ_INSERT_HEAD(&prepareq, newprepare, next);
if (parent)
SIMPLEQ_INSERT_TAIL(&parentq, newparent, next);
if (child)
SIMPLEQ_INSERT_TAIL(&childq, newchild, next);
mutex_unlock(&atfork_lock);
return 0;
}
pid_t
fork(void)
{
struct atfork_callback *iter;
pid_t ret;
mutex_lock(&atfork_lock);
SIMPLEQ_FOREACH(iter, &prepareq, next)
(*iter->fn)();
ret = __fork();
if (ret != 0) {
/*
* We are the parent. It doesn't matter here whether
* the fork call succeeded or failed.
*/
SIMPLEQ_FOREACH(iter, &parentq, next)
(*iter->fn)();
mutex_unlock(&atfork_lock);
} else {
/* We are the child */
SIMPLEQ_FOREACH(iter, &childq, next)
(*iter->fn)();
/*
* Note: We are explicitly *not* unlocking
* atfork_lock. Unlocking atfork_lock is problematic,
* because if any threads in the parent blocked on it
* between the initial lock and the fork() syscall,
* unlocking in the child will try to schedule
* threads, and either the internal mutex interlock or
* the runqueue spinlock could have been held at the
* moment of fork(). Since the other threads do not
* exist in this process, the spinlock will never be
* unlocked, and we would wedge.
* Instead, we reinitialize atfork_lock, since we know
* that the state of the atfork lists is consistent here,
* and that there are no other threads to be affected by
* the forcible cleaning of the queue.
* This permits double-forking to work, although
* it requires knowing that it's "safe" to initialize
* a locked mutex in this context.
*
* The problem exists for users of this interface,
* too, since the intented use of pthread_atfork() is
* to acquire locks across the fork call to ensure
* that the child sees consistent state. There's not
* much that can usefully be done in a child handler,
* and conventional wisdom discourages using them, but
* they're part of the interface, so here we are...
*/
mutex_init(&atfork_lock, NULL);
}
return ret;
}
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