• Linus Torvalds's avatar
    Revert "writeback: plug writeback at a high level" · 0ba13fd1
    Linus Torvalds authored
    This reverts commit d353d758.
    
    Doing the block layer plug/unplug inside writeback_sb_inodes() is
    broken, because that function is actually called with a spinlock held:
    wb->list_lock, as pointed out by Chris Mason.
    
    Chris suggested just dropping and re-taking the spinlock around the
    blk_finish_plug() call (the plgging itself can happen under the
    spinlock), and that would technically work, but is just disgusting.
    
    We do something fairly similar - but not quite as disgusting because we
    at least have a better reason for it - in writeback_single_inode(), so
    it's not like the caller can depend on the lock being held over the
    call, but in this case there just isn't any good reason for that
    "release and re-take the lock" pattern.
    
    [ In general, we should really strive to avoid the "release and retake"
      pattern for locks, because in the general case it can easily cause
      subtle bugs when the caller caches any state around the call that
      might be invalidated by dropping the lock even just temporarily. ]
    
    But in this case, the plugging should be easy to just move up to the
    callers before the spinlock is taken, which should even improve the
    effectiveness of the plug.  So there is really no good reason to play
    games with locking here.
    
    I'll send off a test-patch so that Dave Chinner can verify that that
    plug movement works.  In the meantime this just reverts the problematic
    commit and adds a comment to the function so that we hopefully don't
    make this mistake again.
    Reported-by: default avatarChris Mason <clm@fb.com>
    Cc: Josef Bacik <jbacik@fb.com>
    Cc: Dave Chinner <david@fromorbit.com>
    Cc: Neil Brown <neilb@suse.de>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Christoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0ba13fd1
fs-writeback.c 66.4 KB