This is a note to let you know that I've just added the patch titled
powerpc/vdso: Remove redundant locking in update_vsyscall_tz()
to the 3.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
powerpc-vdso-remove-redundant-locking-in-update_vsyscall_tz.patch
and it can be found in the queue-3.0 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From ce73ec6db47af84d1466402781ae0872a9e7873c Mon Sep 17 00:00:00 2001
From: Shan Hai <[email protected]>
Date: Thu, 8 Nov 2012 15:57:49 +0000
Subject: powerpc/vdso: Remove redundant locking in update_vsyscall_tz()
From: Shan Hai <[email protected]>
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 <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
arch/powerpc/kernel/time.c | 5 -----
1 file changed, 5 deletions(-)
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -859,13 +859,8 @@ void update_vsyscall(struct timespec *wa
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)
Patches currently in stable-queue which might be from [email protected] are
queue-3.0/powerpc-vdso-remove-redundant-locking-in-update_vsyscall_tz.patch
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