[4.2.y-ckt stable] Patch "md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt7.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From f29cb2b6643d86f8533c0e26c5ca27001e6e874c Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb at suse.com>
Date: Wed, 9 Mar 2016 12:58:25 +1100
Subject: md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list

commit 550da24f8d62fe81f3c13e3ec27602d6e44d43dc upstream.

break_stripe_batch_list breaks up a batch and copies some flags from
the batch head to the members, preserving others.

It doesn't preserve or copy STRIPE_PREREAD_ACTIVE.  This is not
normally a problem as STRIPE_PREREAD_ACTIVE is cleared when a
stripe_head is added to a batch, and is not set on stripe_heads
already in a batch.

However there is no locking to ensure one thread doesn't set the flag
after it has just been cleared in another.  This does occasionally happen.

md/raid5 maintains a count of the number of stripe_heads with
STRIPE_PREREAD_ACTIVE set: conf->preread_active_stripes.  When
break_stripe_batch_list clears STRIPE_PREREAD_ACTIVE inadvertently
this could becomes incorrect and will never again return to zero.

md/raid5 delays the handling of some stripe_heads until
preread_active_stripes becomes zero.  So when the above mention race
happens, those stripe_heads become blocked and never progress,
resulting is write to the array handing.

So: change break_stripe_batch_list to preserve STRIPE_PREREAD_ACTIVE
in the members of a batch.

URL: https://bugzilla.kernel.org/show_bug.cgi?id=108741
URL: https://bugzilla.redhat.com/show_bug.cgi?id=1258153
URL: http://thread.gmane.org/5649C0E9.2030204@zoner.cz
Reported-by: Martin Svec <martin.svec at zoner.cz> (and others)
Tested-by: Tom Weber <linux at junkyard.4t2.com>
Fixes: 1b956f7a8f9a ("md/raid5: be more selective about distributing flags across batch.")
Signed-off-by: NeilBrown <neilb at suse.com>
Signed-off-by: Shaohua Li <shli at fb.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.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 71d7cf7..f6be7c4 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -4236,7 +4236,6 @@ static void break_stripe_batch_list(struct stripe_head *head_sh,
 		WARN_ON_ONCE(sh->state & ((1 << STRIPE_ACTIVE) |
 					  (1 << STRIPE_SYNCING) |
 					  (1 << STRIPE_REPLACED) |
-					  (1 << STRIPE_PREREAD_ACTIVE) |
 					  (1 << STRIPE_DELAYED) |
 					  (1 << STRIPE_BIT_DELAY) |
 					  (1 << STRIPE_FULL_WRITE) |
@@ -4251,6 +4250,7 @@ static void break_stripe_batch_list(struct stripe_head *head_sh,
 					      (1 << STRIPE_REPLACED)));

 		set_mask_bits(&sh->state, ~(STRIPE_EXPAND_SYNC_FLAGS |
+					    (1 << STRIPE_PREREAD_ACTIVE) |
 					    (1 << STRIPE_DEGRADED)),
 			      head_sh->state & (1 << STRIPE_INSYNC));

--
2.7.4