fs/block-dev.c:fix performance regression in O_DIRECT writes to md block devices

For regular file, write operaion used blk_plug function.But for block
file,write operation did not use blk_plug.
This patch is also for write-cache mode for block-device.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
vfs: dcache: fix deadlock in tree traversal

IBM reported a deadlock in select_parent().  This was found to be caused
by taking rename_lock when already locked when restarting the tree
traversal.

There are two cases when the traversal needs to be restarted:

 1) concurrent d_move(); this can only happen when not already locked,
    since taking rename_lock protects against concurrent d_move().

 2) racing with final d_put() on child just at the moment of ascending
    to parent; rename_lock doesn't protect against this rare race, so it
    can happen when already locked.

Because of case 2, we need to be able to handle restarting the traversal
when rename_lock is already held.  This patch fixes all three callers of
try_to_ascend().

IBM reported that the deadlock is gone with this patch.

[ I rewrote the patch to be smaller and just do the "goto again" if the
  lock was already held, but credit goes to Miklos for the real work.
   - Linus ]

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>