Re: [PATCH] raid1: include bio_end_io_list in nr_queued to prevent freeze_array hang

[Shaohua Li <shli@kernel.org>]

On Thu, Mar 17, 2016 at 04:29:58PM -0400, Joe Lawrence wrote:
> On 02/29/2016 10:43 AM, Nate Dailey wrote:
> > If raid1d is handling a mix of read and write errors, handle_read_error's
> > call to freeze_array can get stuck.
> > 
> > This can happen because, though the bio_end_io_list is initially drained,
> > writes can be added to it via handle_write_finished as the retry_list
> > is processed. These writes contribute to nr_pending but are not included
> > in nr_queued.
> > 
> > If a later entry on the retry_list triggers a call to handle_read_error,
> > freeze array hangs waiting for nr_pending == nr_queued+extra. The writes
> > on the bio_end_io_list aren't included in nr_queued so the condition will
> > never be satisfied.
> > 
> > To prevent the hang, include bio_end_io_list writes in nr_queued.
> > 
> > There's probably a better way to handle decrementing nr_queued, but this
> > seemed like the safest way to avoid breaking surrounding code.
> > 
> > I'm happy to supply the script I used to repro this hang.
> > 
> > Signed-off-by: Nate Dailey <nate.dailey@stratus.com>
> > ---
> >  drivers/md/raid1.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> > index 4e3843f..bb5bce0 100644
> > --- a/drivers/md/raid1.c
> > +++ b/drivers/md/raid1.c
> > @@ -2274,6 +2274,7 @@ static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
> >  	if (fail) {
> >  		spin_lock_irq(&conf->device_lock);
> >  		list_add(&r1_bio->retry_list, &conf->bio_end_io_list);
> > +		conf->nr_queued++;
> >  		spin_unlock_irq(&conf->device_lock);
> >  		md_wakeup_thread(conf->mddev->thread);
> >  	} else {
> > @@ -2391,8 +2392,10 @@ static void raid1d(struct md_thread *thread)
> >  		LIST_HEAD(tmp);
> >  		spin_lock_irqsave(&conf->device_lock, flags);
> >  		if (!test_bit(MD_CHANGE_PENDING, &mddev->flags)) {
> > -			list_add(&tmp, &conf->bio_end_io_list);
> > -			list_del_init(&conf->bio_end_io_list);
> > +			while (!list_empty(&conf->bio_end_io_list)) {
> > +				list_move(conf->bio_end_io_list.prev, &tmp);
> > +				conf->nr_queued--;
> > +			}
> >  		}
> >  		spin_unlock_irqrestore(&conf->device_lock, flags);
> >  		while (!list_empty(&tmp)) {
> > 
> 
> Nate, Shaohua,
> 
> It looks like bio_end_io_list was added in 55ce74d4bfe1 "md/raid1:
> ensure device failure recorded before write request returns", which
> dates back a ways:
> 
>   % git tag --contains 55ce74d4bfe1b | grep -v 'rc' | sort -V
>   v4.3
>   v4.4
>   v4.5
> 
> Should these patches have 'Fixes' tags for stable backporting?

i'll add it