Re: Deadlock during multipath failover

[Jens Axboe]

On Fri, Feb 13 2009, Christof Schmitt wrote:
> On Thu, Feb 12, 2009 at 12:44:50PM -0800, Mike Anderson wrote:
> > Hannes Reinecke <hare@xxxxxxx> wrote:
> > > Hi Christof,
> > >
> > > Christof Schmitt wrote:
> > >> During failover tests on a current distribution kernel, we found this
> > >> problem. From reading the code, the upstream kernel has the same
> > >> problem:
> > >>
> > >> During multipath failover tests with SCSI on System z, the kernel
> > >> deadlocks in this situation:
> > >>
> > >>>  STACK:
> > >>>  0 blk_add_timer+206 [0x2981ea]
> > >>>  1 blk_rq_timed_out+132 [0x2982a8]
> > >>>  2 blk_abort_request+114 [0x29833e]
> > >>>  3 blk_abort_queue+92 [0x2983a8]
> > >>>  4 deactivate_path+74 [0x3e00009625a]
> > >>>  5 run_workqueue+236 [0x149e04]
> > >>>  6 worker_thread+294 [0x149fce]
> > >>>  7 kthread+110 [0x14f436]
> > >>>  8 kernel_thread_starter+6 [0x10941a]
> > >>
> > >> blk_abort_queue takes the queue_lock with spinlock_irqsave and walks
> > >> the timer_list with list_for_each_entry_safe. Since a path to a SCSI
> > >> device just failed, the rport state is FC_PORTSTATE_BLOCKED. This
> > >> rport state triggers blk_add_timer that calls list_add_tail to move
> > >> the request to the end of timer_list. Thus, the
> > >> list_for_each_entry_safe never reaches the end of the timer_list, it
> > >> continously moves the requests to the end of the list.
> > >>
> > > Hmm. That would be fixes by using list_splice() here:
> > >
> > > diff --git a/block/blk-timeout.c b/block/blk-timeout.c
> > > index a095353..67bcc3f 100644
> > > --- a/block/blk-timeout.c
> > > +++ b/block/blk-timeout.c
> > > @@ -209,12 +209,15 @@ void blk_abort_queue(struct request_queue *q)
> > > {
> > >        unsigned long flags;
> > >        struct request *rq, *tmp;
> > > +       LIST_HEAD(list);
> > >
> > >        spin_lock_irqsave(q->queue_lock, flags);
> > >
> > >        elv_abort_queue(q);
> > >
> > > -       list_for_each_entry_safe(rq, tmp, &q->timeout_list, timeout_list)
> > > +       list_splice_init(&q->timeout_list, &list);
> > > +
> > > +       list_for_each_entry_safe(rq, tmp, &list, timeout_list)
> > >                blk_abort_request(rq);
> > >
> > >        spin_unlock_irqrestore(q->queue_lock, flags);
> > >
> > >> The rport state FC_PORTSTATE_BLOCKED would end, when the function
> > >> fc_timeout_deleted_rport would run to remove the rport. But this
> > >> function was schedules from queue_delayed_work. The timer already
> > >> expired, but the timer function does not run, because the timer
> > >> interrupt is disabled from the spinlock_irqsave call.
> > >>
> > > .. but this shouldn't happen anymore when using splice, as
> > > the timer will be called _after_ the irqrestore above.
> > 
> > If this patch does not address the deadlock another option to look into
> > would be to run some testing without blk_abort_request (just using
> > elv_abort_queue) and not try to abort in flight IOs at this time. 
> > 
> > We observed reduced IO delays during storage failover testing (target
> > responsive but timing out IOs) with this code, but I do not have good
> > breakdown data on the number of IOs handled by elv_abort_queue vs
> > blk_abort_request vs IO delay (It is also config dependent).
> 
> The patch fixes the observed deadlock. While the rport is BLOCKED,
> blk_abort_request only resets the timer for each request, so i would
> guess there is no big difference in calling blk_abort_request or not,
> at least in this scenario.

That approach does look safer to me. Can the original author send a
proper patch so I get attributions correct, then I'll merge it.

-- 
Jens Axboe

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