[ 010/221] powerpc/vdso: Remove redundant locking in update_vsyscall_tz()

Greg Kroah-Hartman
2013-01-15T18:52:24Z

From: Shan Hai <shan.hai@windriver.com>

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/powerpc/kernel/time.c | 5 -----
1 file changed, 5 deletions(-)

--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -774,13 +774,8 @@ void update_vsyscall_old(struct timespec

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)