The unhashed state check in __wait_on_freeing_inode() performed with ->i_lock held against remove_hash_inode() also holding the lock makes another lock acquire in evict() completely spurious -- all potential sleepers already dropped the lock before remove_hash_inode() acquired it or they found the inode to be unhashed and aborted. Note there is no trickery here: the usual cost of both sides taking locks is still being paid, it just stops being paid twice. Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx> --- fs/inode.c | 19 ++++++------------- 1 file changed, 6 insertions(+), 13 deletions(-) diff --git a/fs/inode.c b/fs/inode.c index 10121fc7b87e..4c3be44838a5 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -816,23 +816,16 @@ static void evict(struct inode *inode) /* * Wake up waiters in __wait_on_freeing_inode(). * - * Lockless hash lookup may end up finding the inode before we removed - * it above, but only lock it *after* we are done with the wakeup below. - * In this case the potential waiter cannot safely block. + * It is an invariant that any thread we need to wake up is already + * accounted for before remove_inode_hash() acquires ->i_lock -- both + * sides take the lock and sleep is aborted if the inode is found + * unhashed. Thus either the sleeper wins and goes off CPU, or removal + * wins and the sleeper aborts after testing with the lock. * - * The inode being unhashed after the call to remove_inode_hash() is - * used as an indicator whether blocking on it is safe. + * This also means we don't need any fences for the call below. */ - spin_lock(&inode->i_lock); - /* - * Pairs with the barrier in prepare_to_wait_event() to make sure - * ___wait_var_event() either sees the bit cleared or - * waitqueue_active() check in wake_up_var() sees the waiter. - */ - smp_mb__after_spinlock(); inode_wake_up_bit(inode, __I_NEW); BUG_ON(inode->i_state != (I_FREEING | I_CLEAR)); - spin_unlock(&inode->i_lock); destroy_inode(inode); } -- 2.43.0
