• NeilBrown's avatar
    Remove deadlock potential in md_open · c8c00a69
    NeilBrown authored
    A recent commit:
      commit 449aad3e
    
    introduced the possibility of an A-B/B-A deadlock between
    bd_mutex and reconfig_mutex.
    
    __blkdev_get holds bd_mutex while calling md_open which takes
       reconfig_mutex,
    do_md_run is always called with reconfig_mutex held, and it now
       takes bd_mutex in the call the revalidate_disk.
    
    This potential deadlock was not caught by lockdep due to the
    use of mutex_lock_interruptible_nexted which was introduced
    by
       commit d63a5a74
    do avoid a warning of an impossible deadlock.
    
    It is quite possible to split reconfig_mutex in to two locks.
    One protects the array data structures while it is being
    reconfigured, the other ensures that an array is never even partially
    open while it is being deactivated.
    In particular, the second lock prevents an open from completing
    between the time when do_md_stop checks if there are any active opens,
    and the time when the array is either set read-only, or when ->pers is
    set to NULL.  So we can be certain that no IO is in flight as the
    array is being destroyed.
    
    So create a new lock, open_mutex, just to ensure exclusion between
    'open' and 'stop'.
    
    This avoids the deadlock and also avoids the lockdep warning mentioned
    in commit d63a5a74Reported-by: default avatar"Mike Snitzer" <snitzer@gmail.com>
    Reported-by: default avatar"H. Peter Anvin" <hpa@zytor.com>
    Signed-off-by: default avatarNeilBrown <neilb@suse.de>
    c8c00a69
md.c 178 KB