On (03/21/16 09:06), Byungchul Park wrote:
> On Sun, Mar 20, 2016 at 11:13:10PM +0900, Sergey Senozhatsky wrote:
[..]
> > + if (!sync_print) {
> > + if (in_sched) {
> > + /*
> > + * @in_sched messages may come too early, when we don't
> > + * yet have @printk_kthread. We can't print deferred
> > + * messages directly, because this may deadlock, route
> > + * them via IRQ context.
> > + */
> > + __this_cpu_or(printk_pending,
> > + PRINTK_PENDING_OUTPUT);
> > + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> > + } else if (printk_kthread && !in_panic) {
> > + /* Offload printing to a schedulable context. */
> > + wake_up_process(printk_kthread);
>
> It will not print the "lockup suspected" message at all, for e.g. rq->lock,
> p->pi_lock and any locks which are used within wake_up_process().
this will switch to old SYNC printk() mode should such a lockup ever
happen, which is a giant advantage over any other implementation; doing
wake_up_process() within the 'we can detect recursive printk() here'
gives us better control.
why
printk()->IRQ->wake_up_process()->spin_dump()->printk()->IRQ->wake_up_process()->spin_dump()->printk()->IRQ...
is better?
> Furtheremore, any printk() within wake_up_process() cannot work at all, as
> well.
there is printk_deferred() which has LOGLEVEL_SCHED and which must be used
in sched functions.
-ss
