Re: bcache hangs on writes, recovers after disabling discard on cache device

[Kent Overstreet <kmo-PEzghdH756F8UrSeD/g0lQ@public.gmane.org>]

On Thu, Jul 18, 2013 at 03:05:49PM +0300, Juha Aatrokoski wrote:
> On Tue, 16 Jul 2013, Kent Overstreet wrote:
> 
> >On Tue, Jul 16, 2013 at 09:14:09PM +0300, Juha Aatrokoski wrote:
> >>On Fri, 12 Jul 2013, Juha Aatrokoski wrote:
> >>>>Can you give this patch a try? It's on top of the current
> >>>>bcache-for-3.11 branch
> >>>
> >>>OK, now running the same kernel with this patch applied and
> >>>discard enabled. However, it has previously taken my system 2-4
> >>>days to trigger this bug, so I'd say at least two weeks before I
> >>>can say the patch (may have) fixed the issue.
> >>
> >>No such luck, hit the bug after four days of uptime. Disabling
> >>discard fixed the problem so at least it's not any worse than
> >>before.
> >
> >Argh, damn peculiar bug... and the fact that it takes so long to trigger
> >is frustrating. I'm honestly at a loss at this point as to what that IO
> >actually is.
> 
> One thing I noticed is that your patch only affects the allocator,
> the journal still does discards the old way. Perhaps it's worth a
> try to apply a similar change to the journal discards?

Oh man, thanks for pointing me at that code. This looks like a brown
paper bag bug...

Try this patch and tell me what happens:

From 72c531ee46e73a63739aa3fd10130f167d6bd30d Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kmo-PEzghdH756F8UrSeD/g0lQ@public.gmane.org>
Date: Thu, 18 Jul 2013 10:50:55 -0700
Subject: [PATCH] Fix a dumb journal discard bug


diff --git a/drivers/md/bcache/journal.c b/drivers/md/bcache/journal.c
index ba95ab8..c0017ca 100644
--- a/drivers/md/bcache/journal.c
+++ b/drivers/md/bcache/journal.c
@@ -428,7 +428,7 @@ static void do_journal_discard(struct cache *ca)
 		return;
 	}
 
-	switch (atomic_read(&ja->discard_in_flight) == DISCARD_IN_FLIGHT) {
+	switch (atomic_read(&ja->discard_in_flight)) {
 	case DISCARD_IN_FLIGHT:
 		return;