md/raid5: fix a race condition in stripe batch

We have a race condition in below scenario, say have 3 continuous stripes, sh1,
sh2 and sh3, sh1 is the stripe_head of sh2 and sh3:

CPU1				CPU2				CPU3
handle_stripe(sh3)
				stripe_add_to_batch_list(sh3)
				-> lock(sh2, sh3)
				-> lock batch_lock(sh1)
				-> add sh3 to batch_list of sh1
				-> unlock batch_lock(sh1)
								clear_batch_ready(sh1)
								-> lock(sh1) and batch_lock(sh1)
								-> clear STRIPE_BATCH_READY for all stripes in batch_list
								-> unlock(sh1) and batch_lock(sh1)
->clear_batch_ready(sh3)
-->test_and_clear_bit(STRIPE_BATCH_READY, sh3)
--->return 0 as sh->batch == NULL
				-> sh3->batch_head = sh1
				-> unlock (sh2, sh3)

In CPU1, handle_stripe will continue handle sh3 even it's in batch stripe list
of sh1. By moving sh3->batch_head assignment in to batch_lock, we make it
impossible to clear STRIPE_BATCH_READY before batch_head is set.

Thanks Stephane for helping debug this tricky issue.

Reported-and-tested-by: Stephane Thiell <sthiell@stanford.edu>
Cc: stable@vger.kernel.org (v4.1+)
Signed-off-by: Shaohua Li <shli@fb.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 049a958d3c1e..90414a4e0d49 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -811,6 +811,14 @@ static void stripe_add_to_batch_list(struct r5conf *conf, struct stripe_head *sh
                         spin_unlock(&head->batch_head->batch_lock);
                         goto unlock_out;
                 }
+                /*
+                 * We must assign batch_head of this stripe within the
+                 * batch_lock, otherwise clear_batch_ready of batch head
+                 * stripe could clear BATCH_READY bit of this stripe and
+                 * this stripe->batch_head doesn't get assigned, which
+                 * could confuse clear_batch_ready for this stripe
+                 */
+                sh->batch_head = head->batch_head;
 
                 /*
                  * at this point, head's BATCH_READY could be cleared, but we
@@ -818,8 +826,6 @@ static void stripe_add_to_batch_list(struct r5conf *conf, struct stripe_head *sh
                  */
                 list_add(&sh->batch_list, &head->batch_list);
                 spin_unlock(&head->batch_head->batch_lock);
-
-                sh->batch_head = head->batch_head;
         } else {
                 head->batch_head = head;
                 sh->batch_head = head->batch_head;