Messages in this thread | | | Subject | Re: [PATCH] arm: Fix backtrace generation when IPI is masked | From | Daniel Thompson <> | Date | Tue, 15 Sep 2015 09:10:57 +0100 |
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On 15/09/15 07:58, Hillf Danton wrote: >> Currently on ARM when <SysRq-L> 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 <daniel.thompson at linaro.org> >> --- >> 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.
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