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.