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

[NeilBrown]

On Thu, 29 Jan 2015 12:24:00 +0100 Heinz Mauelshagen <heinzm at redhat.com>
wrote:

> 
> Neil,
> 
> the patch worked fine in overnight test runs without the previous livelock.
> No regressions have been triggered.
> 
> Yes, tidying up that optimization logic (e.g. in fetch_block()) is very 
> much appreciated :-)
> 

Thanks!
The following is what should appear in -next soonish.  If there are any *-by:
tags to be added or changed, please let me know.

NeilBrown


From: NeilBrown <neilb at suse.de>
Date: Mon, 2 Feb 2015 10:44:29 +1100
Subject: [PATCH] md/raid5: fix another livelock caused by non-aligned writes.

If a non-page-aligned write is destined for a device which
is missing/faulty, we can deadlock.

As the target device is missing, a read-modify-write cycle
is not possible.
As the write is not for a full-page, a recontruct-write cycle
is not possible.

This should be handled by logic in fetch_block() which notices
there is a non-R5_OVERWRITE write to a missing device, and so
loads all blocks.

However since commit 67f455486d2ea2, that code requires
STRIPE_PREREAD_ACTIVE before it will active, and those circumstances
never set STRIPE_PREREAD_ACTIVE.

So: in handle_stripe_dirtying, if neither rmw or rcw was possible,
set STRIPE_DELAYED, which will cause STRIPE_PREREAD_ACTIVE be set
after a suitable delay.

Fixes: 67f455486d2ea20b2d94d6adf5b9b783d079e321
Cc: stable at vger.kernel.org (v3.16+)
Reported-by: Mikulas Patocka <mpatocka at redhat.com>
Tested-by: Heinz Mauelshagen <heinzm at redhat.com>
Signed-off-by: NeilBrown <neilb at suse.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 41494d904859..274db1834d43 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3192,6 +3192,11 @@ 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
 	 */
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