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Refactor atfork handlers

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Current implementation (sysdeps/nptl/fork.c) replicates the atfork
handlers list backward to invoke the child handlers after fork/clone
syscall.

The internal atfork handlers is implemented as a single-linked list
so a lock-free algorithm can be used, trading fork mulithread call
performance for some code complexity and dynamic stack allocation
(since the backwards list should not fail).

This patch refactor it to use a dynarary instead of a linked list.
It simplifies the external variables need to be exported and also
the internal atfork handler member definition.

The downside is a serialization of fork call in multithread, since to
operate on the dynarray the internal lock should be used.  However
as noted by Florian, it already acquires external locks for malloc
and libio so it is already hitting some lock contention.  Besides,
posix_spawn should be faster and more scalable to run external programs
in multithread environments.

Checked on x86_64-linux-gnu.

	* nptl/Makefile (routines): Remove unregister-atfork.
	* nptl/register-atfork.c (fork_handler_pool): Remove variable.
	(fork_handler_alloc): Remove function.
	(fork_handlers, fork_handler_init): New variables.
	(__fork_lock): Rename to atfork_lock.
	(__register_atfork, __unregister_atfork, libc_freeres_fn): Rewrite
	to use a dynamic array to add/remove atfork handlers.
	* sysdeps/nptl/fork.c (__libc_fork): Likewise.
	* sysdeps/nptl/fork.h (__fork_lock, __fork_handlers, __linkin_atfork):
	Remove declaration.
	(fork_handler): Remove next, refcntr, and need_signal member.
	(__run_fork_handler_type): New enum.
	(__run_fork_handlers): New prototype.
	* sysdeps/nptl/libc-lockP.h (__libc_atfork): Remove declaration.
This commit is contained in:
Adhemerval Zanella 2018-02-01 17:57:56 -02:00
parent 92aabad9b2
commit 27761a1042
7 changed files with 128 additions and 316 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
ChangeLog
View File

@ -1,5 +1,21 @@
2018-02-22 Adhemerval Zanella <adhemerval.zanella@linaro.org>
* nptl/Makefile (routines): Remove unregister-atfork.
* nptl/register-atfork.c (fork_handler_pool): Remove variable.
(fork_handler_alloc): Remove function.
(fork_handlers, fork_handler_init): New variables.
(__fork_lock): Rename to atfork_lock.
(__register_atfork, __unregister_atfork, libc_freeres_fn): Rewrite
to use a dynamic array to add/remove atfork handlers.
* sysdeps/nptl/fork.c (__libc_fork): Likewise.
* sysdeps/nptl/fork.h (__fork_lock, __fork_handlers, __linkin_atfork):
Remove declaration.
(fork_handler): Remove next, refcntr, and need_signal member.
(__run_fork_handler_type): New enum.
(__run_fork_handlers): New prototype.
* nptl/register-atfork.c: Remove file.
* sysdeps/nptl/libc-lockP.h (__libc_atfork): Remove declaration.
* sysdeps/nptl/nptl-signals.h: Move to ...
* sysdeps/generic/internal-signals.h: ... here. Adjust internal
comments.

2
nptl/Makefile
View File

@ -30,7 +30,7 @@ install-lib-ldscripts := libpthread.so
routines = alloca_cutoff forward libc-lowlevellock libc-cancellation \
libc-cleanup libc_pthread_init libc_multiple_threads \
register-atfork unregister-atfork pthread_self
register-atfork pthread_self
shared-only-routines = forward
# We need to provide certain routines for compatibility with existing

176
nptl/register-atfork.c
View File

@ -22,123 +22,127 @@
#include <fork.h>
#include <atomic.h>
#define DYNARRAY_ELEMENT struct fork_handler
#define DYNARRAY_STRUCT fork_handler_list
#define DYNARRAY_PREFIX fork_handler_list_
#define DYNARRAY_INITIAL_SIZE 48
#include <malloc/dynarray-skeleton.c>
struct fork_handler *__fork_handlers;
/* Lock to protect allocation and deallocation of fork handlers. */
int __fork_lock = LLL_LOCK_INITIALIZER;
/* Number of pre-allocated handler entries. */
#define NHANDLER 48
/* Memory pool for fork handler structures. */
static struct fork_handler_pool
{
struct fork_handler_pool *next;
struct fork_handler mem[NHANDLER];
} fork_handler_pool;
static struct fork_handler *
fork_handler_alloc (void)
{
struct fork_handler_pool *runp = &fork_handler_pool;
struct fork_handler *result = NULL;
unsigned int i;
do
{
/* Search for an empty entry. */
for (i = 0; i < NHANDLER; ++i)
if (runp->mem[i].refcntr == 0)
goto found;
}
while ((runp = runp->next) != NULL);
/* We have to allocate a new entry. */
runp = (struct fork_handler_pool *) calloc (1, sizeof (*runp));
if (runp != NULL)
{
/* Enqueue the new memory pool into the list. */
runp->next = fork_handler_pool.next;
fork_handler_pool.next = runp;
/* We use the last entry on the page. This means when we start
searching from the front the next time we will find the first
entry unused. */
i = NHANDLER - 1;
found:
result = &runp->mem[i];
result->refcntr = 1;
result->need_signal = 0;
}
return result;
}
static struct fork_handler_list fork_handlers;
static bool fork_handler_init = false;
static int atfork_lock = LLL_LOCK_INITIALIZER;
int
__register_atfork (void (*prepare) (void), void (*parent) (void),
void (*child) (void), void *dso_handle)
{
/* Get the lock to not conflict with other allocations. */
lll_lock (__fork_lock, LLL_PRIVATE);
lll_lock (atfork_lock, LLL_PRIVATE);
struct fork_handler *newp = fork_handler_alloc ();
if (!fork_handler_init)
{
fork_handler_list_init (&fork_handlers);
fork_handler_init = true;
}
struct fork_handler *newp = fork_handler_list_emplace (&fork_handlers);
if (newp != NULL)
{
/* Initialize the new record. */
newp->prepare_handler = prepare;
newp->parent_handler = parent;
newp->child_handler = child;
newp->dso_handle = dso_handle;
__linkin_atfork (newp);
}
/* Release the lock. */
lll_unlock (__fork_lock, LLL_PRIVATE);
lll_unlock (atfork_lock, LLL_PRIVATE);
return newp == NULL ? ENOMEM : 0;
}
libc_hidden_def (__register_atfork)
static struct fork_handler *
fork_handler_list_find (struct fork_handler_list *fork_handlers,
void *dso_handle)
{
for (size_t i = 0; i < fork_handler_list_size (fork_handlers); i++)
{
struct fork_handler *elem = fork_handler_list_at (fork_handlers, i);
if (elem->dso_handle == dso_handle)
return elem;
}
return NULL;
}
void
attribute_hidden
__linkin_atfork (struct fork_handler *newp)
__unregister_atfork (void *dso_handle)
{
do
newp->next = __fork_handlers;
while (catomic_compare_and_exchange_bool_acq (&__fork_handlers,
newp, newp->next) != 0);
lll_lock (atfork_lock, LLL_PRIVATE);
struct fork_handler *first = fork_handler_list_find (&fork_handlers,
dso_handle);
/* Removing is done by shifting the elements in the way the elements
that are not to be removed appear in the beginning in dynarray.
This avoid the quadradic run-time if a naive strategy to remove and
shift one element at time. */
if (first != NULL)
{
struct fork_handler *new_end = first;
first++;
for (; first != fork_handler_list_end (&fork_handlers); ++first)
{
if (first->dso_handle != dso_handle)
{
*new_end = *first;
++new_end;
}
}
ptrdiff_t removed = first - new_end;
for (size_t i = 0; i < removed; i++)
fork_handler_list_remove_last (&fork_handlers);
}
lll_unlock (atfork_lock, LLL_PRIVATE);
}
void
__run_fork_handlers (enum __run_fork_handler_type who)
{
struct fork_handler *runp;
if (who == atfork_run_prepare)
{
lll_lock (atfork_lock, LLL_PRIVATE);
size_t sl = fork_handler_list_size (&fork_handlers);
for (size_t i = sl; i > 0; i--)
{
runp = fork_handler_list_at (&fork_handlers, i - 1);
if (runp->prepare_handler != NULL)
runp->prepare_handler ();
}
}
else
{
size_t sl = fork_handler_list_size (&fork_handlers);
for (size_t i = 0; i < sl; i++)
{
runp = fork_handler_list_at (&fork_handlers, i);
if (who == atfork_run_child && runp->child_handler)
runp->child_handler ();
else if (who == atfork_run_parent && runp->parent_handler)
runp->parent_handler ();
}
lll_unlock (atfork_lock, LLL_PRIVATE);
}
}
libc_freeres_fn (free_mem)
{
/* Get the lock to not conflict with running forks. */
lll_lock (__fork_lock, LLL_PRIVATE);
lll_lock (atfork_lock, LLL_PRIVATE);
/* No more fork handlers. */
__fork_handlers = NULL;
fork_handler_list_free (&fork_handlers);
/* Free eventually allocated memory blocks for the object pool. */
struct fork_handler_pool *runp = fork_handler_pool.next;
memset (&fork_handler_pool, '\0', sizeof (fork_handler_pool));
/* Release the lock. */
lll_unlock (__fork_lock, LLL_PRIVATE);
/* We can free the memory after releasing the lock. */
while (runp != NULL)
{
struct fork_handler_pool *oldp = runp;
runp = runp->next;
free (oldp);
}
lll_unlock (atfork_lock, LLL_PRIVATE);
}

121
nptl/unregister-atfork.c
View File

@ -1,121 +0,0 @@
/* Copyright (C) 2002-2018 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
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
<http://www.gnu.org/licenses/>. */
#include <errno.h>
#include <stdlib.h>
#include <fork.h>
#include <atomic.h>
#include <futex-internal.h>
void
__unregister_atfork (void *dso_handle)
{
/* Check whether there is any entry in the list which we have to
remove. It is likely that this is not the case so don't bother
getting the lock.
We do not worry about other threads adding entries for this DSO
right this moment. If this happens this is a race and we can do
whatever we please. The program will crash anyway seen. */
struct fork_handler *runp = __fork_handlers;
struct fork_handler *lastp = NULL;
while (runp != NULL)
if (runp->dso_handle == dso_handle)
break;
else
{
lastp = runp;
runp = runp->next;
}
if (runp == NULL)
/* Nothing to do. */
return;
/* Get the lock to not conflict with additions or deletions. Note
that there couldn't have been another thread deleting something.
The __unregister_atfork function is only called from the
dlclose() code which itself serializes the operations. */
lll_lock (__fork_lock, LLL_PRIVATE);
/* We have to create a new list with all the entries we don't remove. */
struct deleted_handler
{
struct fork_handler *handler;
struct deleted_handler *next;
} *deleted = NULL;
/* Remove the entries for the DSO which is unloaded from the list.
It's a single linked list so readers are. */
do
{
again:
if (runp->dso_handle == dso_handle)
{
if (lastp == NULL)
{
/* We have to use an atomic operation here because
__linkin_atfork also uses one. */
if (catomic_compare_and_exchange_bool_acq (&__fork_handlers,
runp->next, runp)
!= 0)
{
runp = __fork_handlers;
goto again;
}
}
else
lastp->next = runp->next;
/* We cannot overwrite the ->next element now. Put the deleted
entries in a separate list. */
struct deleted_handler *newp = alloca (sizeof (*newp));
newp->handler = runp;
newp->next = deleted;
deleted = newp;
}
else
lastp = runp;
runp = runp->next;
}
while (runp != NULL);
/* Release the lock. */
lll_unlock (__fork_lock, LLL_PRIVATE);
/* Walk the list of all entries which have to be deleted. */
while (deleted != NULL)
{
/* We need to be informed by possible current users. */
deleted->handler->need_signal = 1;
/* Make sure this gets written out first. */
atomic_write_barrier ();
/* Decrement the reference counter. If it does not reach zero
wait for the last user. */
atomic_decrement (&deleted->handler->refcntr);
unsigned int val;
while ((val = deleted->handler->refcntr) != 0)
futex_wait_simple (&deleted->handler->refcntr, val, FUTEX_PRIVATE);
deleted = deleted->next;
}
}

96
sysdeps/nptl/fork.c
View File

@ -48,11 +48,6 @@ pid_t
__libc_fork (void)
{
pid_t pid;
struct used_handler
{
struct fork_handler *handler;
struct used_handler *next;
} *allp = NULL;
/* Determine if we are running multiple threads. We skip some fork
handlers in the single-thread case, to make fork safer to use in
@ -60,60 +55,7 @@ __libc_fork (void)
but our current fork implementation is not. */
bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
/* Run all the registered preparation handlers. In reverse order.
While doing this we build up a list of all the entries. */
struct fork_handler *runp;
while ((runp = __fork_handlers) != NULL)
{
/* Make sure we read from the current RUNP pointer. */
atomic_full_barrier ();
unsigned int oldval = runp->refcntr;
if (oldval == 0)
/* This means some other thread removed the list just after
the pointer has been loaded. Try again. Either the list
is empty or we can retry it. */
continue;
/* Bump the reference counter. */
if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
oldval + 1, oldval))
/* The value changed, try again. */
continue;
/* We bumped the reference counter for the first entry in the
list. That means that none of the following entries will
just go away. The unloading code works in the order of the
list.
While executing the registered handlers we are building a
list of all the entries so that we can go backward later on. */
while (1)
{
/* Execute the handler if there is one. */
if (runp->prepare_handler != NULL)
runp->prepare_handler ();
/* Create a new element for the list. */
struct used_handler *newp
= (struct used_handler *) alloca (sizeof (*newp));
newp->handler = runp;
newp->next = allp;
allp = newp;
/* Advance to the next handler. */
runp = runp->next;
if (runp == NULL)
break;
/* Bump the reference counter for the next entry. */
atomic_increment (&runp->refcntr);
}
/* We are done. */
break;
}
__run_fork_handlers (atfork_run_prepare);
/* If we are not running multiple threads, we do not have to
preserve lock state. If fork runs from a signal handler, only
@ -198,29 +140,7 @@ __libc_fork (void)
__rtld_lock_initialize (GL(dl_load_lock));
/* Run the handlers registered for the child. */
while (allp != NULL)
{
if (allp->handler->child_handler != NULL)
allp->handler->child_handler ();
/* Note that we do not have to wake any possible waiter.
This is the only thread in the new process. The count
may have been bumped up by other threads doing a fork.
We reset it to 1, to avoid waiting for non-existing
thread(s) to release the count. */
allp->handler->refcntr = 1;
/* XXX We could at this point look through the object pool
and mark all objects not on the __fork_handlers list as
unused. This is necessary in case the fork() happened
while another thread called dlclose() and that call had
to create a new list. */
allp = allp->next;
}
/* Initialize the fork lock. */
__fork_lock = LLL_LOCK_INITIALIZER;
__run_fork_handlers (atfork_run_child);
}
else
{
@ -235,17 +155,7 @@ __libc_fork (void)
}
/* Run the handlers registered for the parent. */
while (allp != NULL)
{
if (allp->handler->parent_handler != NULL)
allp->handler->parent_handler ();
if (atomic_decrement_and_test (&allp->handler->refcntr)
&& allp->handler->need_signal)
futex_wake (&allp->handler->refcntr, 1, FUTEX_PRIVATE);
allp = allp->next;
}
__run_fork_handlers (atfork_run_parent);
}
return pid;

31
sysdeps/nptl/fork.h
View File

@ -24,29 +24,37 @@ extern unsigned long int __fork_generation attribute_hidden;
/* Pointer to the fork generation counter in the thread library. */
extern unsigned long int *__fork_generation_pointer attribute_hidden;
/* Lock to protect allocation and deallocation of fork handlers. */
extern int __fork_lock attribute_hidden;
/* Elements of the fork handler lists. */
struct fork_handler
{
struct fork_handler *next;
void (*prepare_handler) (void);
void (*parent_handler) (void);
void (*child_handler) (void);
void *dso_handle;
unsigned int refcntr;
int need_signal;
};
/* The single linked list of all currently registered for handlers. */
extern struct fork_handler *__fork_handlers attribute_hidden;
/* Function to call to unregister fork handlers. */
extern void __unregister_atfork (void *dso_handle) attribute_hidden;
#define UNREGISTER_ATFORK(dso_handle) __unregister_atfork (dso_handle)
enum __run_fork_handler_type
{
atfork_run_prepare,
atfork_run_child,
atfork_run_parent
};
/* Run the atfork handlers and lock/unlock the internal lock depending
of the WHO argument:
- atfork_run_prepare: run all the PREPARE_HANDLER in reverse order of
insertion and locks the internal lock.
- atfork_run_child: run all the CHILD_HANDLER and unlocks the internal
lock.
- atfork_run_parent: run all the PARENT_HANDLER and unlocks the internal
lock. */
extern void __run_fork_handlers (enum __run_fork_handler_type who)
attribute_hidden;
/* C library side function to register new fork handlers. */
extern int __register_atfork (void (*__prepare) (void),
@ -54,6 +62,3 @@ extern int __register_atfork (void (*__prepare) (void),
void (*__child) (void),
void *dso_handle);
libc_hidden_proto (__register_atfork)
/* Add a new element to the fork list. */
extern void __linkin_atfork (struct fork_handler *newp) attribute_hidden;

2
sysdeps/nptl/libc-lockP.h
View File

@ -319,8 +319,6 @@ __libc_cleanup_routine (struct __pthread_cleanup_frame *f)
/* Register handlers to execute before and after `fork'. Note that the
last parameter is NULL. The handlers registered by the libc are
never removed so this is OK. */
#define __libc_atfork(PREPARE, PARENT, CHILD) \
__register_atfork (PREPARE, PARENT, CHILD, NULL)
extern int __register_atfork (void (*__prepare) (void),
void (*__parent) (void),
void (*__child) (void),