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 <stable@vger.kernel.org> know about it. >From ce73ec6db47af84d1466402781ae0872a9e7873c Mon Sep 17 00:00:00 2001 From: Shan Hai <shan....@windriver.com> Date: Thu, 8 Nov 2012 15:57:49 +0000 Subject: powerpc/vdso: Remove redundant locking in update_vsyscall_tz() From: Shan Hai <shan....@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....@windriver.com> Signed-off-by: Benjamin Herrenschmidt <b...@kernel.crashing.org> Signed-off-by: Greg Kroah-Hartman <gre...@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 @@ -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 shan....@windriver.com are queue-3.0/powerpc-vdso-remove-redundant-locking-in-update_vsyscall_tz.patch -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html