Patch "md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
md-raid5-preserve-stripe_preread_active-in-break_stripe_batch_list.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: NeilBrown <[email protected]>
Date: Wed, 9 Mar 2016 12:58:25 +1100
Subject: md/raid5: preserve STRIPE_PREREAD_ACTIVE in break_stripe_batch_list

From: NeilBrown <[email protected]>

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/[email protected]

---
 drivers/md/raid5.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -4241,7 +4241,6 @@ static void break_stripe_batch_list(stru
 		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) |

@@ -4256,6 +4255,7 @@ static void break_stripe_batch_list(stru
 					      (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));
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/raid5-check_reshape-shouldn-t-call-mddev_suspend.patch queue-4.4/md-raid5-preserve-stripe_preread_active-in-break_stripe_batch_list.patch