From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail1.windriver.com (mail1.windriver.com [147.11.146.13]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (Client CN "mail1.windriver.com", Issuer "Intel External Basic Issuing CA 3A" (not verified)) by ozlabs.org (Postfix) with ESMTPS id 813BB2C0087 for ; Wed, 14 Nov 2012 20:39:39 +1100 (EST) Date: Wed, 14 Nov 2012 17:39:15 +0800 From: Shan Hai To: Subject: Re: [PATCH 0/1] powerpc/vdso: remove redundant locking in update_vsyscall_tz() Message-ID: <20121114093914.GA14139@debian.corp.ad.wrs.com> References: <20121109015718.GA6124@debian.corp.ad.wrs.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" In-Reply-To: <20121109015718.GA6124@debian.corp.ad.wrs.com> Cc: linuxppc-dev@lists.ozlabs.org, john.stultz@linaro.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Nov 09, 2012 at 09:57:49AM +0800, Shan Hai wrote: > 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(). > > --- > arch/powerpc/kernel/time.c | 5 ----- > 1 file changed, 5 deletions(-) > Hi Ben, Would you please review this one? >>From 167eac293b07e0ee201ffe5043ec442d52495a48 Mon Sep 17 00:00:00 2001 From: Shan Hai Date: Mon, 5 Nov 2012 18:24:22 +0800 Subject: [PATCH 1/1] powerpc/vdso: remove redundant locking in 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 --- arch/powerpc/kernel/time.c | 5 ----- 1 file changed, 5 deletions(-) diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c index 03b29a6..426141f 100644 --- a/arch/powerpc/kernel/time.c +++ b/arch/powerpc/kernel/time.c @@ -859,13 +859,8 @@ void update_vsyscall(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) -- 1.7.10.4 Best Regards Shan Hai > Thanks > Shan Hai > From 167eac293b07e0ee201ffe5043ec442d52495a48 Mon Sep 17 00:00:00 2001 > From: Shan Hai > Date: Mon, 5 Nov 2012 18:24:22 +0800 > Subject: [PATCH 1/1] powerpc/vdso: remove redundant locking in > 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 > --- > arch/powerpc/kernel/time.c | 5 ----- > 1 file changed, 5 deletions(-) > > diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c > index 03b29a6..426141f 100644 > --- a/arch/powerpc/kernel/time.c > +++ b/arch/powerpc/kernel/time.c > @@ -859,13 +859,8 @@ void update_vsyscall(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) > -- > 1.7.10.4 > > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev