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Re: [PATCH] nptl: Avoid fork handler lock for async-signal-safe fork [BZ #24161]
* Adhemerval Zanella:
>> void
>> -__run_fork_handlers (enum __run_fork_handler_type who)
>> +__run_fork_handlers (enum __run_fork_handler_type who, _Bool multiple_threads)
>
> I think we should rename to enable_lock or something similar.
Please consider the patch below.
>> diff --git a/sysdeps/nptl/fork.c b/sysdeps/nptl/fork.c
>> index bd68f18b45..14b69a6f89 100644
>> --- a/sysdeps/nptl/fork.c
>> +++ b/sysdeps/nptl/fork.c
>> @@ -55,7 +55,7 @@ __libc_fork (void)
>> but our current fork implementation is not. */
>> bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
>
> We have the macro SINGLE_THREAD_P to select the best strategy depending
> of the architecture/ABI.
Is this a suggestion for a separate change?
Thanks,
Florian
nptl: Avoid fork handler lock for async-signal-safe fork [BZ #24161]
Commit 27761a1042daf01987e7d79636d0c41511c6df3c ("Refactor atfork
handlers") introduced a lock, atfork_lock, around fork handler list
accesses. It turns out that this lock occasionally results in
self-deadlocks in malloc/tst-mallocfork2:
(gdb) bt
#0 __lll_lock_wait_private ()
at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:63
#1 0x00007f160c6f927a in __run_fork_handlers (who=(unknown: 209394016),
who@entry=atfork_run_prepare) at register-atfork.c:116
#2 0x00007f160c6b7897 in __libc_fork () at ../sysdeps/nptl/fork.c:58
#3 0x00000000004027d6 in sigusr1_handler (signo=<optimized out>)
at tst-mallocfork2.c:80
#4 sigusr1_handler (signo=<optimized out>) at tst-mallocfork2.c:64
#5 <signal handler called>
#6 0x00007f160c6f92e4 in __run_fork_handlers (who=who@entry=atfork_run_parent)
at register-atfork.c:136
#7 0x00007f160c6b79a2 in __libc_fork () at ../sysdeps/nptl/fork.c:152
#8 0x0000000000402567 in do_test () at tst-mallocfork2.c:156
#9 0x0000000000402dd2 in support_test_main (argc=1, argv=0x7ffc81ef1ab0,
config=config@entry=0x7ffc81ef1970) at support_test_main.c:350
#10 0x0000000000402362 in main (argc=<optimized out>, argv=<optimized out>)
at ../support/test-driver.c:168
If no locking happens in the single-threaded case (where fork is
expected to be async-signal-safe), this deadlock is avoided.
(pthread_atfork is not required to be async-signal-safe, so a fork
call from a signal handler interrupting pthread_atfork is not
a problem.)
2019-02-04 Florian Weimer <fweimer@redhat.com>
[BZ #24161]
* sysdeps/nptl/fork.h (__run_fork_handlers): Add multiple_threads
argument.
* nptl/register-atfork.c (__run_fork_handlers): Only perform
locking if the new multiple_threads argument is true.
* sysdeps/nptl/fork.c (__libc_fork): Pass multiple_threads to
__run_fork_handlers.
diff --git a/nptl/register-atfork.c b/nptl/register-atfork.c
index bc797b761a..80a1becb5f 100644
--- a/nptl/register-atfork.c
+++ b/nptl/register-atfork.c
@@ -107,13 +107,14 @@ __unregister_atfork (void *dso_handle)
}
void
-__run_fork_handlers (enum __run_fork_handler_type who)
+__run_fork_handlers (enum __run_fork_handler_type who, _Bool do_locking)
{
struct fork_handler *runp;
if (who == atfork_run_prepare)
{
- lll_lock (atfork_lock, LLL_PRIVATE);
+ if (do_locking)
+ lll_lock (atfork_lock, LLL_PRIVATE);
size_t sl = fork_handler_list_size (&fork_handlers);
for (size_t i = sl; i > 0; i--)
{
@@ -133,7 +134,8 @@ __run_fork_handlers (enum __run_fork_handler_type who)
else if (who == atfork_run_parent && runp->parent_handler)
runp->parent_handler ();
}
- lll_unlock (atfork_lock, LLL_PRIVATE);
+ if (do_locking)
+ lll_unlock (atfork_lock, LLL_PRIVATE);
}
}
diff --git a/sysdeps/nptl/fork.c b/sysdeps/nptl/fork.c
index bd68f18b45..14b69a6f89 100644
--- a/sysdeps/nptl/fork.c
+++ b/sysdeps/nptl/fork.c
@@ -55,7 +55,7 @@ __libc_fork (void)
but our current fork implementation is not. */
bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
- __run_fork_handlers (atfork_run_prepare);
+ __run_fork_handlers (atfork_run_prepare, multiple_threads);
/* If we are not running multiple threads, we do not have to
preserve lock state. If fork runs from a signal handler, only
@@ -134,7 +134,7 @@ __libc_fork (void)
__rtld_lock_initialize (GL(dl_load_lock));
/* Run the handlers registered for the child. */
- __run_fork_handlers (atfork_run_child);
+ __run_fork_handlers (atfork_run_child, multiple_threads);
}
else
{
@@ -149,7 +149,7 @@ __libc_fork (void)
}
/* Run the handlers registered for the parent. */
- __run_fork_handlers (atfork_run_parent);
+ __run_fork_handlers (atfork_run_parent, multiple_threads);
}
return pid;
diff --git a/sysdeps/nptl/fork.h b/sysdeps/nptl/fork.h
index a1c3b26b68..99ed76034b 100644
--- a/sysdeps/nptl/fork.h
+++ b/sysdeps/nptl/fork.h
@@ -52,9 +52,11 @@ enum __run_fork_handler_type
- 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;
+ lock.
+
+ Perform locking only if DO_LOCKING. */
+extern void __run_fork_handlers (enum __run_fork_handler_type who,
+ _Bool do_locking) attribute_hidden;
/* C library side function to register new fork handlers. */
extern int __register_atfork (void (*__prepare) (void),