Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753884AbdGJLXb (ORCPT ); Mon, 10 Jul 2017 07:23:31 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:34414 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752666AbdGJLX3 (ORCPT ); Mon, 10 Jul 2017 07:23:29 -0400 Date: Mon, 10 Jul 2017 12:22:29 +0100 From: Mark Rutland To: Ding Tianhong , Marc Zyngier , Daniel Lezcano , Thomas Gleixner Cc: Catalin Marinas , Will Deacon , LinuxArm , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH RESEND] arm64: arch_timer: fix the infinite recursion when enable ftrace and erratum workaround Message-ID: <20170710112229.GA18040@leverpostej> References: <321e4c16-aa14-beee-b6dc-36e19e5ec35a@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <321e4c16-aa14-beee-b6dc-36e19e5ec35a@huawei.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2995 Lines: 80 On Sun, Jul 09, 2017 at 04:30:54PM +0800, Ding Tianhong wrote: > When enable preempt and debug ftrace, and perform the following steps, the > system will hang: > mount -t debugfs nodev /sys/kernel/debug/ > cd /sys/kernel/debug/tracing/ > echo function_graph > current_tracer > > This is because tracing the preempt_disable/enable calls would cause > trace_clock() which would get local timer to go into infinite recursion > when enable the arch timer erratum workaround for some chips, so Prevent > tracing of preempt_disable/enable() in arch_timer_reg_read_stable(). > > This problem is similar to the fixed by upstream commit 96b3d28bf4 > ("sched/clock: Prevent tracing recursion in sched_clock_cpu()"). As I mentioned before, the patch itself looks fine to me, but the commit message is somewhat difficult to read. Can we please change this to: arm64: arch_timer: avoid infinite recursion when ftrace is enabled On platforms with an arch timer erratum workaround, it's possible for arch_timer_reg_read_stable() to recurse into itself when certain tracing options are enabled, leading to stack overflows and related problems. For example, when PREEMPT_TRACER and FUNCTION_GRAPH_TRACER are selected, it's possible to trigger this with: $ mount -t debugfs nodev /sys/kernel/debug/ $ echo function_graph > /sys/kernel/debug/tracing/current_tracer The problem is that in such cases, preempt_disable() instrumentation attempts to acquire a timestamp via trace_clock(), resulting in a call back to arch_timer_reg_read_stable(), and hence recursion. This patch changes arch_timer_reg_read_stable() to use preempt_{disable,enable}_notrace(), which avoids this. With that commit message: Acked-by: Mark Rutland Daniel, Thomas, would you be happy to fold that in when picking this? Or would you prefer that I fix this up and resend? Thanks, Mark. > Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs") > Signed-off-by: Ding Tianhong > --- > arch/arm64/include/asm/arch_timer.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h > index 74d08e4..67bb7a4 100644 > --- a/arch/arm64/include/asm/arch_timer.h > +++ b/arch/arm64/include/asm/arch_timer.h > @@ -65,13 +65,13 @@ struct arch_timer_erratum_workaround { > u64 _val; \ > if (needs_unstable_timer_counter_workaround()) { \ > const struct arch_timer_erratum_workaround *wa; \ > - preempt_disable(); \ > + preempt_disable_notrace(); \ > wa = __this_cpu_read(timer_unstable_counter_workaround); \ > if (wa && wa->read_##reg) \ > _val = wa->read_##reg(); \ > else \ > _val = read_sysreg(reg); \ > - preempt_enable(); \ > + preempt_enable_notrace(); \ > } else { \ > _val = read_sysreg(reg); \ > } \ > -- > 1.9.0 > >