Re: [PATCH v2 0/2] getrusage: use sig->stats_lock

[Oleg Nesterov <oleg@redhat.com>]

On 01/23, Dylan Hatch wrote:
>
> I have one last question -- is there possibly an edge case in which
> the hard lockup
> can still happen? How likely is it for many writers to force enough
> readers to do a
> retry on the seqlock, disabling irq and causing the lockup?

I don't know how likely is it, and I guess the repro should be more creative ;)

But yes. Please see the TODO: section in the changelog,

	- Turn sig->stats_lock into seqcount_rwlock_t, this way the
	  readers in the slow mode won't exclude each other.

and more importantly,

	- stats_lock has to disable irqs because ->siglock can be taken
	  in irq context, it would be very nice to change __exit_signal()
	  to avoid the siglock->stats_lock dependency.

There are other users which take stats_lock under siglock (and the
"fs/proc: do_task_stat" series changes 2 of them to not do this), but
__exit_signal() is most problematic.

If we remove this dependency, we can turn read_seqbegin_or_lock_irqsave()
into read_seqbegin_or_lock() which doesn't disable irqs.

Oleg.