Re: [PATCH next] sbitmap: fix lockup while swapping

[Hugh Dickins <hughd@google.com>]

On Tue, 27 Sep 2022, Jan Kara wrote:
> On Mon 26-09-22 20:39:03, Hugh Dickins wrote:
> 
> So my thinking was that instead of having multiple counters, we'd have just
> two - one counting completions and the other one counting wakeups and if
> completions - wakeups > batch, we search for waiters in the wait queues,
> wake them up so that 'wakeups' counter catches up. That also kind of
> alleviates the 'wake_index' issue because racing updates to it will lead to
> reordering of wakeups but not to lost wakeups, retries, or anything.
> 
> I also agree with your wake_up_nr_return() idea below, that is part of this
> solution (reliably waking given number of waiters) and in fact I have
> already coded that yesterday while thinking about the problem ;)

Great - I'm pleasantly surprised to have been not so far off,
and we seem to be much in accord.

(What I called wake_up_nr_return() can perfectly well be wake_up_nr()
itself: I had just been temporarily avoiding a void to int change in
a header file, recompiling the world.)

Many thanks for your detailed elucidation of the batch safety,
in particular: I won't pretend to have absorbed it completely yet,
but it's there in your mail for me and all of us to refer back to.

> > TBH I have not tested this one outside of that experiment: would you
> > prefer this patch to my first one, I test and sign this off and send?
> 
> Yes, actually this is an elegant solution. It has the same inherent
> raciness as your waitqueue_active() patch so wakeups could be lost even
> though some waiters need them but that seems pretty unlikely. So yes, if
> you can submit this, I guess this is a good band aid for the coming merge
> window.

No problem in the testing, the v2 patch follows now.

> 
> > > 2) Revert Yu Kuai's original fix 040b83fcecfb8 ("sbitmap: fix possible io
> > > hung due to lost wakeup") and my fixup 48c033314f37 ("sbitmap: Avoid leaving
> > > waitqueue in invalid state in __sbq_wake_up()"). But then Keith would have
> > > to redo his batched accounting patches on top.
> > 
> > I know much too little to help make that choice.
> 
> Yeah, I guess it is Jens' call in the end. I'm fine with both options.
> 
> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR