Commit 6d183de4 authored Nov 28, 2013 by NeilBrown

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



commit 566c09c5 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: 566c09c5
Signed-off-by: NeilBrown <neilb@suse.de>

parent 142d44c3

drivers/md/raid5.c

+4 −7

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