This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
[PATCH]: Fix blocking pthread_join.
- From: Stefan Liebler <stli at linux dot vnet dot ibm dot com>
- To: GNU C Library <libc-alpha at sourceware dot org>
- Date: Wed, 25 Apr 2018 13:27:07 +0200
- Subject: [PATCH]: Fix blocking pthread_join.
Hi,
On s390 (31bit) if glibc is build with -Os, pthread_join sometimes
blocks indefinitely. This is e.g. observable with
testcase intl/tst-gettext6.
pthread_join is calling lll_wait_tid(tid), which performs the futex-wait
syscall in a loop as long as tid != 0 (thread is alive).
On s390 (and build with -Os), tid is loaded from memory before
comparing against zero and then the tid is loaded a second time
in order to pass it to the futex-wait-syscall.
If the thread exits in between, then the futex-wait-syscall is
called with the value zero and it waits until a futex-wake occurs.
As the thread is already exited, there won't be a futex-wake.
In lll_wait_tid, the tid is stored to the local variable __tid,
which is then used as argument for the futex-wait-syscall.
But unfortunately the compiler is allowed to reload the value
from memory.
With this patch, the tid is loaded by dereferencing a volatile pointer.
Then the compiler is not allowed to reload the value for __tid from memory.
Okay to commit?
Bye
Stefan
---
ChangeLog:
* sysdeps/nptl/lowlevellock.h (lll_wait_tid):
Use a volatile pointer to load __tid.
commit be2e80e32fa4d0c7f7d021f550d21ab102aa8c42
Author: Stefan Liebler <stli@linux.vnet.ibm.com>
Date: Wed Apr 25 12:51:27 2018 +0200
Fix blocking pthread_join.
On s390 (31bit) if glibc is build with -Os, pthread_join sometimes
blocks indefinitely. This is e.g. observable with
testcase intl/tst-gettext6.
pthread_join is calling lll_wait_tid(tid), which performs the futex-wait
syscall in a loop as long as tid != 0 (thread is alive).
On s390 (and build with -Os), tid is loaded from memory before
comparing against zero and then the tid is loaded a second time
in order to pass it to the futex-wait-syscall.
If the thread exits in between, then the futex-wait-syscall is
called with the value zero and it waits until a futex-wake occurs.
As the thread is already exited, there won't be a futex-wake.
In lll_wait_tid, the tid is stored to the local variable __tid,
which is then used as argument for the futex-wait-syscall.
But unfortunately the compiler is allowed to reload the value
from memory.
With this patch, the tid is loaded by dereferencing a volatile pointer.
Then the compiler is not allowed to reload the value for __tid from memory.
ChangeLog:
* sysdeps/nptl/lowlevellock.h (lll_wait_tid):
Use a volatile pointer to load __tid.
diff --git a/sysdeps/nptl/lowlevellock.h b/sysdeps/nptl/lowlevellock.h
index 8326e2805c..6e06e8498a 100644
--- a/sysdeps/nptl/lowlevellock.h
+++ b/sysdeps/nptl/lowlevellock.h
@@ -184,7 +184,7 @@ extern int __lll_timedlock_wait (int *futex, const struct timespec *,
#define lll_wait_tid(tid) \
do { \
__typeof (tid) __tid; \
- while ((__tid = (tid)) != 0) \
+ while ((__tid = *(volatile __typeof(tid) *) &(tid)) != 0) \
lll_futex_wait (&(tid), __tid, LLL_SHARED);\
} while (0)