md/raid5: fix newly-broken locking in get_active_stripe.

commit 566c09c53455d7c4f1 raid5: relieve lock contention in get_active_stripe()

modified the locking in get_active_stripe() reducing the range
protected by the (highly contended) device_lock.
Unfortunately it reduced the range too much opening up some races.

One race can occur if get_priority_stripe runs between the
test on sh->count and device_lock being taken.
This will mean that sh->lru is not empty while get_active_stripe
thinks ->count is zero resulting in a 'BUG' firing.

Another race happens if __release_stripe is called immediately
after sh->count is tested and found to be non-zero.  If STRIPE_HANDLE
is not set, get_active_stripe should increment ->active_stripes
when it increments ->count from 0, but as it didn't think it was 0,
it doesn't.

Extending device_lock to cover the test on sh->count close these
races.

While we are here, fix the two BUG tests:
 -If count is zero, then lru really must not be empty, or we've
  lock the stripe_head somehow - no other tests are relevant.
 -STRIPE_ON_RELEASE_LIST is completely independent of ->lru so
  testing it is pointless.

Reported-and-tested-by: Brassow Jonathan <jbrassow@redhat.com>
Reviewed-by: Shaohua Li <shli@kernel.org>
Fixes: 566c09c53455d7c4f1
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -678,26 +678,23 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
 			} else
 				init_stripe(sh, sector, previous);
 		} else {
+			spin_lock(&conf->device_lock);
 			if (atomic_read(&sh->count)) {
 				BUG_ON(!list_empty(&sh->lru)
 				    && !test_bit(STRIPE_EXPANDING, &sh->state)
 				    && !test_bit(STRIPE_ON_UNPLUG_LIST, &sh->state)
-				    && !test_bit(STRIPE_ON_RELEASE_LIST, &sh->state));
+					);
 			} else {
-				spin_lock(&conf->device_lock);
 				if (!test_bit(STRIPE_HANDLE, &sh->state))
 					atomic_inc(&conf->active_stripes);
-				if (list_empty(&sh->lru) &&
-				    !test_bit(STRIPE_ON_RELEASE_LIST, &sh->state) &&
-				    !test_bit(STRIPE_EXPANDING, &sh->state))
-					BUG();
+				BUG_ON(list_empty(&sh->lru));
 				list_del_init(&sh->lru);
 				if (sh->group) {
 					sh->group->stripes_cnt--;
 					sh->group = NULL;
 				}
-				spin_unlock(&conf->device_lock);
 			}
+			spin_unlock(&conf->device_lock);
 		}
 	} while (sh == NULL);