On (05/29/18 11:51), Hoeun Ryu wrote:
> Make printk_safe_flush() safe in NMI context.
> nmi_trigger_cpumask_backtrace() can be called in NMI context. For example the
> function is called in watchdog_overflow_callback() if the flag of hardlockup
> backtrace (sysctl_hardlockup_all_cpu_backtrace) is true and
> watchdog_overflow_callback() function is called in NMI context on some
> architectures.
> Calling printk_safe_flush() in nmi_trigger_cpumask_backtrace() eventually
> tries
> to lock logbuf_lock in vprintk_emit() but the logbuf_lock can be already
> locked in
> preempted contexts (task or irq in this case) or by other CPUs and it may
> cause
> deadlocks.
> By making printk_safe_flush() safe in NMI context, the backtrace triggering
> CPU
> just skips flushing if the lock is not avaiable in NMI context. The messages
> in
> per-cpu nmi buffer of the backtrace triggering CPU can be lost if the CPU is
> in
> hard lockup (because irq is disabled here) but if panic() is not called. The
> flushing can be delayed by the next irq work in normal cases.
Any chance we can add more info to the commit message? E.g. backtraces
which would describe "how" is this possible (like the one I posted in
another email). Just to make it more clear.
> @@ -254,6 +254,16 @@ void printk_safe_flush(void)
> {
> int cpu;
>
> + /*
> + * Just avoid deadlocks here, we could loose the messages in per-cpu
> nmi buffer
> + * in the case that hardlockup happens but panic() is not called
> (irq_work won't
> + * work).
> + * The flushing can be delayed by the next irq_work if flushing is
> skippped here
^^
skipped
-ss
