On Mon, 25 May 2015 14:46:27 +0200 Petr Mladek <pmla...@suse.cz> wrote:
> It might take ages until users see messages from NMI context. They cannot
> be flushed to the console because the operation involves taking and
> releasing a bunch of locks. Everything gets fixed by the followup printk
> in normal context but it is not predictable.
>
> The same problem has printk_sched() and this patch reuses the existing
> solution.
>
> There is no special printk() variant for NMI context. Hence the IRQ work
> need to get queued from vprintk_emit().
>
> ...
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1554,9 +1554,6 @@ int printk_deferred(const char *fmt, ...)
> va_start(args, fmt);
> r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
> va_end(args);
> -
> - __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> preempt_enable();
>
> return r;
> @@ -1880,7 +1877,10 @@ asmlinkage int vprintk_emit(int facility, int level,
> * If called from the scheduler or NMI context, we can not get console
> * without a possible deadlock.
> */
> - if (!in_sched && !in_nmi()) {
> + if (in_sched || in_nmi()) {
> + __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> + } else {
This looks hairy. Why irq_work_queue() OK to call from NMI?
arch/arm64/kernel/smp.c uses smp_cross_call() which might use NMI!
Presumably it'll call directly if the target CPU==this_cpu but I didn't
run around and audit everything.
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