[dm-devel] [PATCH] md: fix raid5 livelock

[Jes Sorensen]

NeilBrown <neilb at suse.de> writes:
> On Sun, 25 Jan 2015 21:06:20 +0100 Heinz Mauelshagen <heinzm at redhat.com>
> wrote:
>
>> From: Heinz Mauelshagen <heinzm at 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 at redhat.com>
>> Tested-by: Jon Brassow <jbrassow at redhat.com>
>> Tested-by: Heinz Mauelshagen <heinzm at 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!!!

Neil,

Since this one seems to do the trick, will you be pushing it into you
tree anytime soon?

Cheers,
Jes