Re: [PATCH v2] printk: Avoid softlockups in console_unlock()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  4 Feb 2013 23:17:10 +0100
Jan Kara <jack@suse.cz> wrote:

> A CPU can be caught in console_unlock() for a long time (tens of seconds are
> reported by our customers) when other CPUs are using printk heavily and serial
> console makes printing slow. Despite serial console drivers are calling
> touch_nmi_watchdog() this triggers softlockup warnings because
> interrupts are disabled for the whole time console_unlock() runs (e.g.
> vprintk() calls console_unlock() with interrupts disabled). Thus IPIs
> cannot be processed and other CPUs get stuck spinning in calls like
> smp_call_function_many(). Also RCU eventually starts reporting lockups.
> 
> In my artifical testing I also managed to trigger a situation when disk
> disappeared from the system apparently because commands to / from it
> could not be delivered for long enough. This is why just silencing
> watchdogs isn't a reliable solution to the problem and we simply have to
> avoid spending too long in console_unlock().
> 
> We fix the issue by limiting the time we spend in console_unlock() to
> watchdog_thresh() / 4 (unless we are in an early boot stage or oops is
> happening). The rest of the buffer will be printed either by further
> callers to printk() or by a queued work.

I still hate the patch :(

> ...
>
> +void console_unlock(void)
> +{
> +	if (__console_unlock()) {
> +		/* Let worker do the rest of printing */
> +		schedule_work(&printk_work);
> +	}
>  }

This creates another place from where we cannot call printk(): anywhere
where worker_pool.lock is held.

And as schedule_work() can do a wakeup it creates a third reason why
the sched code cannot call printk (along with rq->lock taken by
wake_up(klogd) and rq->lock taken by up(&console_sem).  Hence
printk_sched().  See the lkml thread "[GIT PULL] printk: Support for
full dynticks mode".

We already have machinery for doing async tickling in printk: the
printk_pending stuff.  Did you consider adding another
PRINTK_PENDING_foo in some fashion?