Patch "md/raid5: preserve STRIPE_ON_UNPLUG_LIST in break_stripe_batch_list" has been added to the 4.9-stable tree

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

md/raid5: preserve STRIPE_ON_UNPLUG_LIST in break_stripe_batch_list

to the 4.9-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_on_unplug_list-in-break_stripe_batch_list.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Dennis Yang <[email protected]>
Date: Wed, 6 Sep 2017 11:02:35 +0800
Subject: md/raid5: preserve STRIPE_ON_UNPLUG_LIST in break_stripe_batch_list

From: Dennis Yang <[email protected]>

commit 184a09eb9a2fe425e49c9538f1604b05ed33cfef upstream.

In release_stripe_plug(), if a stripe_head has its STRIPE_ON_UNPLUG_LIST set, it indicates that this stripe_head is already in the raid5_plug_cb list and release_stripe() would be called instead to drop a reference count. Otherwise, the STRIPE_ON_UNPLUG_LIST bit would be set for this stripe_head and it will get queued into the raid5_plug_cb list.

Since break_stripe_batch_list() did not preserve STRIPE_ON_UNPLUG_LIST, A stripe could be re-added to plug list while it is still on that list in the following situation. If stripe_head A is added to another stripe_head B's batch list, in this case A will have its batch_head != NULL and be added into the plug list. After that, stripe_head B gets handled and called break_stripe_batch_list() to reset all the batched stripe_head(including A which is still on the plug list)'s state and reset their batch_head to NULL. Before the plug list gets processed, if there is another write request comes in and get stripe_head A, A will have its batch_head == NULL (cleared by calling break_stripe_batch_list() on B) and be added to plug list once again.

---
 drivers/md/raid5.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -4283,7 +4283,8 @@ static void break_stripe_batch_list(stru
 
 		set_mask_bits(&sh->state, ~(STRIPE_EXPAND_SYNC_FLAGS |
 					    (1 << STRIPE_PREREAD_ACTIVE) |
-					    (1 << STRIPE_DEGRADED)),
+					    (1 << STRIPE_DEGRADED) |
+					    (1 << STRIPE_ON_UNPLUG_LIST)),
 			      head_sh->state & (1 << STRIPE_INSYNC));
 
 		sh->check_state = head_sh->check_state;

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

queue-4.9/md-raid5-preserve-stripe_on_unplug_list-in-break_stripe_batch_list.patch