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); }
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