On Thu, 06 Jun 2013 09:00:26 -0600 Tregaron Bayly <tbayly@xxxxxxxxxxxx> wrote: > On 06/03/2013 07:49 PM, NeilBrown wrote: > > On Sun, 2 Jun 2013 15:43:41 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> > > wrote: > > > >> Hello Neil, > >> I believe I have found what is causing the deadlock. It happens in two flavors: > >> > >> 1) > >> # raid1d() is called, and conf->pending_bio_list is non-empty at this point > >> # raid1d() calls md_check_recovery(), which eventually calls > >> raid1_add_disk(), which calls raise_barrier() > >> # Now raise_barrier will wait for conf->nr_pending to become 0, but it > >> cannot become 0, because there are bios sitting in > >> conf->pending_bio_list, which nobody will flush, because raid1d is the > >> one supposed to call flush_pending_writes(), either directly or via > >> handle_read_error. But it is stuck in raise_barrier. > >> > >> 2) > >> # raid1_add_disk() calls raise_barrier(), and waits for > >> conf->nr_pending to become 0, as before > >> # new WRITE comes and calls wait_barrier(), but this thread has a > >> non-empty current->bio_list > >> # In this case, the code allows the WRITE to go through > >> wait_barrier(), and trigger WRITEs to mirror legs, but these WRITEs > >> again end up in conf->pending_bio_list (either via raid1_unplug or > >> directly). But nobody will flush conf->pending_bio_list, because > >> raid1d is stuck in raise_barrier. > >> > >> Previously, for example in kernel 3.2, raid1_add_disk did not call > >> raise_barrier, so this problem did not happen. > >> > >> Attached is a reproduction with some prints that I added to > >> raise_barrier and wait_barrier (their code also attached). It > >> demonstrates case 2. It shows that once raise_barrier got called, > >> conf->nr_pending drops down, until it equals the number of > >> wait_barrier calls, that slipped through because of non-empty > >> current->bio_list. And at this point, this array hangs. > >> > >> Can you please comment on how to fix this problem. It looks like a > >> real deadlock. > >> We can perhaps call md_check_recovery() after flush_pending_writes(), > >> but this only makes the window smaller, not closes it entirely. But it > >> looks like we really should not be calling raise_barrier from raid1d. > >> > >> Thanks, > >> Alex. > > > > Hi Alex, > > thanks for the analysis. > > > > Does the following patch fix it? It makes raise_barrier more like > > freeze_array(). > > If not, could you try making the same change to the first > > wait_event_lock_irq in raise_barrier? > > > > Thanks. > > NeilBrown > > > > > > > > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c > > index 328fa2d..d34f892 100644 > > --- a/drivers/md/raid1.c > > +++ b/drivers/md/raid1.c > > @@ -828,9 +828,9 @@ static void raise_barrier(struct r1conf *conf) > > conf->barrier++; > > > > /* Now wait for all pending IO to complete */ > > - wait_event_lock_irq(conf->wait_barrier, > > - !conf->nr_pending && conf->barrier < RESYNC_DEPTH, > > - conf->resync_lock); > > + wait_event_lock_irq_cmd(conf->wait_barrier, > > + !conf->nr_pending && conf->barrier < RESYNC_DEPTH, > > + conf->resync_lock, flush_pending_writes(conf)); > > > > spin_unlock_irq(&conf->resync_lock); > > } > > > > Neil, > > This deadlock also cropped up in 3.4 between .37 and .38. Passing flush_pending_writes(conf) as cmd to wait_event_lock_irq seems to fix it there as well. > > --- linux-3.4.38/drivers/md/raid1.c 2013-03-28 13:12:41.000000000 -0600 > +++ linux-3.4.38.patch/drivers/md/raid1.c 2013-06-04 12:17:35.314194903 -0600 > @@ -751,7 +751,7 @@ > /* Now wait for all pending IO to complete */ > wait_event_lock_irq(conf->wait_barrier, > !conf->nr_pending && conf->barrier < RESYNC_DEPTH, > - conf->resync_lock, ); > + conf->resync_lock, flush_pending_writes(conf)); > > spin_unlock_irq(&conf->resync_lock); > } > I suspect it was already there in 3.4.37 .. in fact I think it was there in 3.4. I'll mark the fix for including in 3.4-stable Thanks for the report. NeilBrown
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