From mboxrd@z Thu Jan 1 00:00:00 1970 From: daniel.thompson@linaro.org (Daniel Thompson) Date: Tue, 15 Sep 2015 09:10:57 +0100 Subject: [PATCH] arm: Fix backtrace generation when IPI is masked In-Reply-To: <017001d0ef83$e18a5d00$a49f1700$@alibaba-inc.com> References: <016701d0ef82$c303d6a0$490b83e0$@alibaba-inc.com> <017001d0ef83$e18a5d00$a49f1700$@alibaba-inc.com> Message-ID: <55F7D291.5090509@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 15/09/15 07:58, Hillf Danton wrote: >> Currently on ARM when is triggered from an interrupt handler >> (e.g. a SysRq issued using UART or kbd) the main CPU will wedge for ten >> seconds with interrupts masked before issuing a backtrace for every CPU >> except itself. >> >> The new backtrace code introduced by commit 96f0e00378d4 ("ARM: add >> basic support for on-demand backtrace of other CPUs") does not work >> correctly when run from an interrupt handler because IPI_CPU_BACKTRACE >> is used to generate the backtrace on all CPUs but cannot preempt the >> current calling context. >> >> This can be fixed by detecting that the calling context cannot be >> preempted and issuing the backtrace directly in this case. Some small >> changes to the generic code are required to support this. >> >> Signed-off-by: Daniel Thompson >> --- >> arch/arm/kernel/smp.c | 7 +++++++ >> lib/nmi_backtrace.c | 5 ++++- >> 2 files changed, 11 insertions(+), 1 deletion(-) >> >> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c >> index 48185a773852..4d8a80328c74 100644 >> --- a/arch/arm/kernel/smp.c >> +++ b/arch/arm/kernel/smp.c >> @@ -748,6 +748,13 @@ core_initcall(register_cpufreq_notifier); >> >> static void raise_nmi(cpumask_t *mask) >> { >> + /* >> + * Generate the backtrace directly if we are running in a >> + * calling context that is not preemptible by the backtrace IPI. >> + */ >> + if (cpumask_test_cpu(smp_processor_id(), mask) && irqs_disabled()) >> + nmi_cpu_backtrace(NULL); >> + >> smp_cross_call(mask, IPI_CPU_BACKTRACE); >> } >> >> diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c >> index 88d3d32e5923..be0466a80d0b 100644 >> --- a/lib/nmi_backtrace.c >> +++ b/lib/nmi_backtrace.c >> @@ -149,7 +149,10 @@ bool nmi_cpu_backtrace(struct pt_regs *regs) >> /* Replace printk to write into the NMI seq */ >> this_cpu_write(printk_func, nmi_vprintk); >> pr_warn("NMI backtrace for cpu %d\n", cpu); >> - show_regs(regs); >> + if (regs) >> + show_regs(regs); >> + else >> + dump_stack(); > > Better if dump_stack() is added in a separate patch, given that > it is not mentioned in commit message. Adding dump_stack() is mentioned in passing ("Some small changes to the generic code are required to support this.") but you're right that the reason for the change is not explicitly called out. I can certainly respin as two patches but perhaps its better just to improve the commit message. Something like: > This can be fixed by detecting that the calling context cannot be > preempted and issuing the backtrace directly in this case. Issuing > directly leaves us without any pt_regs to pass to nmi_cpu_backtrace(). > Modify the generic code to call dump_stack() when its argument is > NULL. Which do you prefer? Daniel. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752706AbbIOILJ (ORCPT ); Tue, 15 Sep 2015 04:11:09 -0400 Received: from mail-wi0-f177.google.com ([209.85.212.177]:34638 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751478AbbIOILA (ORCPT ); Tue, 15 Sep 2015 04:11:00 -0400 Subject: Re: [PATCH] arm: Fix backtrace generation when IPI is masked To: Hillf Danton References: <016701d0ef82$c303d6a0$490b83e0$@alibaba-inc.com> <017001d0ef83$e18a5d00$a49f1700$@alibaba-inc.com> Cc: linux-kernel , linux-arm-kernel@lists.infradead.org From: Daniel Thompson Message-ID: <55F7D291.5090509@linaro.org> Date: Tue, 15 Sep 2015 09:10:57 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <017001d0ef83$e18a5d00$a49f1700$@alibaba-inc.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/09/15 07:58, Hillf Danton wrote: >> Currently on ARM when is triggered from an interrupt handler >> (e.g. a SysRq issued using UART or kbd) the main CPU will wedge for ten >> seconds with interrupts masked before issuing a backtrace for every CPU >> except itself. >> >> The new backtrace code introduced by commit 96f0e00378d4 ("ARM: add >> basic support for on-demand backtrace of other CPUs") does not work >> correctly when run from an interrupt handler because IPI_CPU_BACKTRACE >> is used to generate the backtrace on all CPUs but cannot preempt the >> current calling context. >> >> This can be fixed by detecting that the calling context cannot be >> preempted and issuing the backtrace directly in this case. Some small >> changes to the generic code are required to support this. >> >> Signed-off-by: Daniel Thompson >> --- >> arch/arm/kernel/smp.c | 7 +++++++ >> lib/nmi_backtrace.c | 5 ++++- >> 2 files changed, 11 insertions(+), 1 deletion(-) >> >> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c >> index 48185a773852..4d8a80328c74 100644 >> --- a/arch/arm/kernel/smp.c >> +++ b/arch/arm/kernel/smp.c >> @@ -748,6 +748,13 @@ core_initcall(register_cpufreq_notifier); >> >> static void raise_nmi(cpumask_t *mask) >> { >> + /* >> + * Generate the backtrace directly if we are running in a >> + * calling context that is not preemptible by the backtrace IPI. >> + */ >> + if (cpumask_test_cpu(smp_processor_id(), mask) && irqs_disabled()) >> + nmi_cpu_backtrace(NULL); >> + >> smp_cross_call(mask, IPI_CPU_BACKTRACE); >> } >> >> diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c >> index 88d3d32e5923..be0466a80d0b 100644 >> --- a/lib/nmi_backtrace.c >> +++ b/lib/nmi_backtrace.c >> @@ -149,7 +149,10 @@ bool nmi_cpu_backtrace(struct pt_regs *regs) >> /* Replace printk to write into the NMI seq */ >> this_cpu_write(printk_func, nmi_vprintk); >> pr_warn("NMI backtrace for cpu %d\n", cpu); >> - show_regs(regs); >> + if (regs) >> + show_regs(regs); >> + else >> + dump_stack(); > > Better if dump_stack() is added in a separate patch, given that > it is not mentioned in commit message. Adding dump_stack() is mentioned in passing ("Some small changes to the generic code are required to support this.") but you're right that the reason for the change is not explicitly called out. I can certainly respin as two patches but perhaps its better just to improve the commit message. Something like: > This can be fixed by detecting that the calling context cannot be > preempted and issuing the backtrace directly in this case. Issuing > directly leaves us without any pt_regs to pass to nmi_cpu_backtrace(). > Modify the generic code to call dump_stack() when its argument is > NULL. Which do you prefer? Daniel.