From: Shan Hai Date: Thu, 8 Nov 2012 15:57:49 +0000 (+0000) Subject: powerpc/vdso: Remove redundant locking in update_vsyscall_tz() X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=ce73ec6db47a;p=linux-beck.git 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 CC: Signed-off-by: Benjamin Herrenschmidt --- diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c index b3b14352b05e..6f6b1cccc916 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)