From 3664847d95e60a9a943858b7800f8484669740fc Mon Sep 17 00:00:00 2001 From: Shaohua Li Date: Fri, 25 Aug 2017 10:40:02 -0700 Subject: [PATCH] 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 Cc: stable@vger.kernel.org (v4.1+) Signed-off-by: Shaohua Li --- drivers/md/raid5.c | 10 ++++++++-- 1 file 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;