Messages in this thread | | | Date | Wed, 27 May 2015 16:14:23 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 04/10] printk: Merge and flush NMI buffer predictably via IRQ work |
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On Mon, 25 May 2015 14:46:27 +0200 Petr Mladek <pmladek@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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