Commit 3664847d authored Aug 25, 2017 by Shaohua Li

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>

parent e8a27f83

drivers/md/raid5.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -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;