Messages in this thread | | | Subject | Re: [PATCH v2] arm: Fix backtrace generation when IPI is masked | From | Daniel Thompson <> | Date | Tue, 15 Sep 2015 14:15:10 +0100 |
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On 15/09/15 12:30, Russell King - ARM Linux wrote: > On Tue, Sep 15, 2015 at 12:05:12PM +0100, Daniel Thompson 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 patch needs a little more work - what happens to the IPI_CPU_BACKTRACE > we've sent to ourselves? (It fires after the interrupt handler for the > UART/kbd has finished.) It ought to be masked out if we're going to > handle it a different way.
Actually it already gets masked out. The argument to raise_nmi() points to a data structure owned by the backtrace library functions and this structure if altered during the execution of nmi_cpu_backtrace() to clear the calling CPU.
I had originally planned to use cpumask_test_and_clear_cpu() for the conditional branch but that would be broken because nmi_cpu_backtrace() would become a nop if we clear anything from the mask before calling it!
I guess I should add a comment about this to save us from broken but "obviously correct" cleanups in the future...
Daniel.
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