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 <email@example.com> Reviewed-by: Shaohua Li <firstname.lastname@example.org> Fixes: 566c09c5 Signed-off-by: NeilBrown <email@example.com>