From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Glibc hackers <libc-hacker@sources.redhat.com>
Subject: [PATCH] endless loop in __libc_fork
Date: Thu, 11 Sep 2008 15:07:00 -0000 [thread overview]
Message-ID: <1221144185.14064.1.camel@localhost> (raw)
Greetings,
Christian Borntraeger found a bug in __libc_fork with a KVM stress on s390:
pid_t
__libc_fork (void)
{
...
/* 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)
{
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;
...
}
The (oldval == 0) check with the continue is translated to an endless
loop on s390-64:
16: e3 60 10 00 00 04 lg %r6,0(%r1)
1c: b9 02 00 66 ltgr %r6,%r6 # runp != NULL check
20: b9 04 00 bf lgr %r11,%r15
24: 41 40 60 28 la %r4,40(%r6)
28: a7 74 00 9f jne 166 <__libc_fork+0x166>
...
166: 58 30 60 28 l %r3,40(%r6)
> 16a: 12 33 ltr %r3,%r3 # oldval == 0 check
> 16c: a7 84 ff ff je 16a <__libc_fork+0x16a> # endless loop
Once you stumbled over the problem it is obvious that a memory barrier
is missing here. See patch below.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
--
2008-09-11 Martin Schwidefsky <schwidefsky@de.ibm.com>
* sysdeps/unix/sysv/linux/fork.c (__libc_fork): Add memory barrier
to force runp->refcntr to be read from memory.
diff -urpN libc/nptl/sysdeps/unix/sysv/linux/fork.c libc-s390/nptl/sysdeps/unix/sysv/linux/fork.c
--- libc/nptl/sysdeps/unix/sysv/linux/fork.c 2007-08-07 13:09:01.000000000 +0200
+++ libc-s390/nptl/sysdeps/unix/sysv/linux/fork.c 2008-09-08 13:20:13.000000000 +0200
@@ -64,7 +64,11 @@ __libc_fork (void)
struct fork_handler *runp;
while ((runp = __fork_handlers) != NULL)
{
- unsigned int oldval = runp->refcntr;
+ unsigned int oldval;
+
+ atomic_full_barrier();
+
+ oldval = runp->refcntr;
if (oldval == 0)
/* This means some other thread removed the list just after
next reply other threads:[~2008-09-11 15:07 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-09-11 15:07 Martin Schwidefsky [this message]
2008-10-06 16:47 ` Ulrich Drepper
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