• Dmitry Monakhov's avatar
    ext4: completed_io locking cleanup · 28a535f9
    Dmitry Monakhov authored
    Current unwritten extent conversion state-machine is very fuzzy.
    - For unknown reason it performs conversion under i_mutex. What for?
      My diagnosis:
      We already protect extent tree with i_data_sem, truncate and punch_hole
      should wait for DIO, so the only data we have to protect is end_io->flags
      modification, but only flush_completed_IO and end_io_work modified this
      flags and we can serialize them via i_completed_io_lock.
      Currently all these games with mutex_trylock result in the following deadlock
       truncate:                          kworker:
        ext4_setattr                       ext4_end_io_work
        inode_dio_wait(inode)  ->BLOCK
                                 DEADLOCK<- mutex_trylock()
      unlink $MNT/file
      fallocate -l $((1024*1024*1024)) $MNT/file
      aio-stress -I 100000 -O -s 100m -n -t 1 -c 10 -o 2 -o 3 $MNT/file
      sleep 2
      truncate -s 0 $MNT/file
    Or use 286's xfstests https://github.com/dmonakhov/xfstests/blob/devel/286
    This patch makes state machine simple and clean:
    (1) xxx_end_io schedule final extent conversion simply by calling
        ext4_add_complete_io(), which append it to ei->i_completed_io_list
        NOTE1: because of (2A) work should be queued only if
        ->i_completed_io_list was empty, otherwise the work is scheduled already.
    (2) ext4_flush_completed_IO is responsible for handling all pending
        end_io from ei->i_completed_io_list
        Flushing sequence consists of following stages:
        A) LOCKED: Atomically drain completed_io_list to local_list
        B) Perform extents conversion
        C) LOCKED: move converted io's to to_free list for final deletion
           	     This logic depends on context which we was called from.
        D) Final end_io context destruction
        NOTE1: i_mutex is no longer required because end_io->flags modification
        is protected by ei->ext4_complete_io_lock
    Full list of changes:
    - Move all completion end_io related routines to page-io.c in order to improve
      logic locality
    - Move open coded logic from various xx_end_xx routines to ext4_add_complete_io()
    - remove EXT4_IO_END_FSYNC
    - Improve SMP scalability by removing useless i_mutex which does not
      protect io->flags anymore.
    - Reduce lock contention on i_completed_io_lock by optimizing list walk.
    - Rename ext4_end_io_nolock to end4_end_io and make it static
    - Check flush completion status to ext4_ext_punch_hole(). Because it is
      not good idea to punch blocks from corrupted inode.
    Changes since V3 (in request to Jan's comments):
      Fall back to active flush_completed_IO() approach in order to prevent
      performance issues with nolocked DIO reads.
    Changes since V2:
      Fix use-after-free caused by race truncate vs end_io_work
    Signed-off-by: default avatarDmitry Monakhov <dmonakhov@openvz.org>
    Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>