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authorShan Hai <shan.hai@windriver.com>2012-11-08 15:57:49 +0000
committerBenjamin Herrenschmidt <benh@kernel.crashing.org>2013-01-03 16:45:51 +1100
commitce73ec6db47af84d1466402781ae0872a9e7873c (patch)
tree05929e9c525a6a5ef7a46795499f5b34dbbddf8b /arch/powerpc/kernel
parentd1c3ed669a2d452cacfb48c2d171a1f364dae2ed (diff)
downloadlinaro-lsk-ce73ec6db47af84d1466402781ae0872a9e7873c.tar.gz
powerpc/vdso: Remove redundant locking in update_vsyscall_tz()
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> CC: <stable@vger.kernel.org> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Diffstat (limited to 'arch/powerpc/kernel')
-rw-r--r--arch/powerpc/kernel/time.c5
1 files changed, 0 insertions, 5 deletions
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index b3b14352b05..6f6b1cccc91 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -770,13 +770,8 @@ void update_vsyscall_old(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)