Re: [PATCH] md: fix raid5 livelock

[NeilBrown <neilb@suse.de>]

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On Sun, 25 Jan 2015 21:06:20 +0100 Heinz Mauelshagen <heinzm@redhat.com>
wrote:

> From: Heinz Mauelshagen <heinzm@redhat.com>
> 
> Hi Neil,
> 
> the reconstruct write optimization in raid5, function fetch_block causes
> livelocks in LVM raid4/5 tests.
> 
> Test scenarios:
> the tests wait for full initial array resynchronization before making a 
> filesystem
> on the raid4/5 logical volume, mounting it, writing to the filesystem 
> and failing
> one physical volume holding a raiddev.
> 
> In short, we're seeing livelocks on fully synchronized raid4/5 arrays 
> with a failed device.
> 
> This patch fixes the issue but likely in a suboptimnal way.
> 
> Do you think there is a better solution to avoid livelocks on 
> reconstruct writes?
> 
> Regards,
> Heinz
> 
> Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
> Tested-by: Jon Brassow <jbrassow@redhat.com>
> Tested-by: Heinz Mauelshagen <heinzm@redhat.com>
> 
> ---
>   drivers/md/raid5.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index c1b0d52..0fc8737 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -2915,7 +2915,7 @@ static int fetch_block(struct stripe_head *sh, 
> struct stripe_head_state *s,
>               (s->failed >= 1 && fdev[0]->toread) ||
>               (s->failed >= 2 && fdev[1]->toread) ||
>               (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
> -             (!test_bit(R5_Insync, &dev->flags) || 
> test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) &&
> +             (!test_bit(R5_Insync, &dev->flags) || 
> test_bit(STRIPE_PREREAD_ACTIVE, &sh->state) || s->non_overwrite) &&
>                !test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||
>               ((sh->raid_conf->level == 6 ||
>                 sh->sector >= sh->raid_conf->mddev->recovery_cp)


That is a bit heavy handed, but knowing that fixes the problem helps a lot.

I think the problem happens when processes a non-overwrite write to a failed
device.

fetch_block() should, in that case, pre-read all of the working device, but
since

	      (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) &&

was added, it sometimes doesn't.  The root problem is that
handle_stripe_dirtying is getting confused because neither rmw or rcw seem to
work, so it doesn't start the chain of events to set STRIPE_PREREAD_ACTIVE.

The following (which is against mainline) might fix it.  Can you test?

Thanks,
NeilBrown

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index c1b0d52bfcb0..793cf2861e97 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3195,6 +3195,10 @@ static void handle_stripe_dirtying(struct r5conf *conf,
 					  (unsigned long long)sh->sector,
 					  rcw, qread, test_bit(STRIPE_DELAYED, &sh->state));
 	}
+	if (rcw > disks && rmw > disks &&
+	    !test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
+		set_bit(STRIPE_DELAYED, &sh->state);
+
 	/* now if nothing is locked, and if we have enough data,
 	 * we can start a write request
 	 */


This code really really needs to be tidied up and commented better!!!

Thanks,
NeilBrown


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