From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tregaron Bayly Subject: Re: BUG - raid 1 deadlock on handle_read_error / wait_barrier Date: Thu, 06 Jun 2013 09:00:26 -0600 Message-ID: <51B0A40A.2010208@bluehost.com> References: <1361487504.4863.54.camel@linux-lxtg.site> <20130225094350.4b8ef084@notabene.brown> <20130225110458.2b1b1e2d@notabene.brown> <1361808662.20264.4.camel@148> <20130520171753.002f07d9@notabene.brown> <20130604114924.37e4573c@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130604114924.37e4573c@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Alexander Lyakas , linux-raid , Shyam Kaushik , yair@zadarastorage.com List-Id: linux-raid.ids On 06/03/2013 07:49 PM, NeilBrown wrote: > On Sun, 2 Jun 2013 15:43:41 +0300 Alexander Lyakas > 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); } Thanks, Tregaron