md/raid5: fix locking in handle_stripe_clean_event()

After commit 566c09c53455 ("raid5: relieve lock contention in get_active_stripe()")
__find_stripe() is called under conf->hash_locks + hash.
But handle_stripe_clean_event() calls remove_hash() under
conf->device_lock.

Under some cirscumstances the hash chain can be circuited,
and we get an infinite loop with disabled interrupts and locked hash
lock in __find_stripe(). This leads to hard lockup on multiple CPUs
and following system crash.

I was able to reproduce this behavior on raid6 over 6 ssd disks.
The devices_handle_discard_safely option should be set to enable trim
support. The following script was used:

for i in `seq 1 32`; do
    dd if=/dev/zero of=large$i bs=10M count=100 &
done

neilb: original was against a 3.x kernel.  I forward-ported
  to 4.3-rc.  This verison is suitable for any kernel since
  Commit: 59fc630b8b5f ("RAID5: batch adjacent full stripe write")
  (v4.1+).  I'll post a version for earlier kernels to stable.

Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
Fixes: 566c09c53455 ("raid5: relieve lock contention in get_active_stripe()")
Signed-off-by: NeilBrown <neilb@suse.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: <stable@vger.kernel.org> # 3.13 - 4.2

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 49bb8d3ff9be..45933c160697 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3499,6 +3499,7 @@ returnbi:
                 }
         if (!discard_pending &&
             test_bit(R5_Discard, &sh->dev[sh->pd_idx].flags)) {
+                int hash;
                 clear_bit(R5_Discard, &sh->dev[sh->pd_idx].flags);
                 clear_bit(R5_UPTODATE, &sh->dev[sh->pd_idx].flags);
                 if (sh->qd_idx >= 0) {
@@ -3512,16 +3513,17 @@ returnbi:
                  * no updated data, so remove it from hash list and the stripe
                  * will be reinitialized
                  */
-                spin_lock_irq(&conf->device_lock);
 unhash:
+                hash = sh->hash_lock_index;
+                spin_lock_irq(conf->hash_locks + hash);
                 remove_hash(sh);
+                spin_unlock_irq(conf->hash_locks + hash);
                 if (head_sh->batch_head) {
                         sh = list_first_entry(&sh->batch_list,
                                               struct stripe_head, batch_list);
                         if (sh != head_sh)
                                         goto unhash;
                 }
-                spin_unlock_irq(&conf->device_lock);
                 sh = head_sh;
 
                 if (test_bit(STRIPE_SYNC_REQUESTED, &sh->state))