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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
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2
sysdeps/nptl/libc-lockP.h
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@@ -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),