diff options
| author | Shan Hai <shan.hai@windriver.com> | 2012-11-08 15:57:49 +0000 |
|---|---|---|
| committer | Ben Hutchings <ben@decadent.org.uk> | 2013-01-16 01:13:16 +0000 |
| commit | 615df262c1ab4fbec7e198c5c956a7bca6ef7f22 (patch) | |
| tree | 432697c03c3631349fa4cb8acf313e9318ceaf9f | |
| parent | 2fe324eaf2ae6a91126c2951f35718e392d6f043 (diff) | |
powerpc/vdso: Remove redundant locking in update_vsyscall_tz()
commit ce73ec6db47af84d1466402781ae0872a9e7873c upstream.
The locking in update_vsyscall_tz() is not only unnecessary because the vdso
code copies the data unproteced in __kernel_gettimeofday() but also
introduces a hard to reproduce race condition between update_vsyscall()
and update_vsyscall_tz(), which causes user space process to loop
forever in vdso code.
The following patch removes the locking from update_vsyscall_tz().
Locking is not only unnecessary because the vdso code copies the data
unprotected in __kernel_gettimeofday() but also erroneous because updating
the tb_update_count is not atomic and introduces a hard to reproduce race
condition between update_vsyscall() and update_vsyscall_tz(), which further
causes user space process to loop forever in vdso code.
The below scenario describes the race condition,
x==0 Boot CPU other CPU
proc_P: x==0
timer interrupt
update_vsyscall
x==1 x++;sync settimeofday
update_vsyscall_tz
x==2 x++;sync
x==3 sync;x++
sync;x++
proc_P: x==3 (loops until x becomes even)
Because the ++ operator would be implemented as three instructions and not
atomic on powerpc.
A similar change was made for x86 in commit 6c260d58634
("x86: vdso: Remove bogus locking in update_vsyscall_tz")
Signed-off-by: Shan Hai <shan.hai@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
| -rw-r--r-- | arch/powerpc/kernel/time.c | 5 |
1 files changed, 0 insertions, 5 deletions
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c index ec8affecf94..e74f86e3f9a 100644 --- a/arch/powerpc/kernel/time.c +++ b/arch/powerpc/kernel/time.c @@ -859,13 +859,8 @@ void update_vsyscall(struct timespec *wall_time, struct timespec *wtm, void update_vsyscall_tz(void) { - /* Make userspace gettimeofday spin until we're done. */ - ++vdso_data->tb_update_count; - smp_mb(); vdso_data->tz_minuteswest = sys_tz.tz_minuteswest; vdso_data->tz_dsttime = sys_tz.tz_dsttime; - smp_mb(); - ++vdso_data->tb_update_count; } static void __init clocksource_init(void) |