| From | Greg Kroah-Hartman <> | Subject | [ 010/221] powerpc/vdso: Remove redundant locking in update_vsyscall_tz() | Date | Tue, 15 Jan 2013 10:48:57 -0800 |
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3.7-stable review patch. If anyone has any objections, please let me know.
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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)
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