• Al Viro's avatar
    livelock avoidance in sget() · acfec9a5
    Al Viro authored
    Eric Sandeen has found a nasty livelock in sget() - take a mount(2) about
    to fail.  The superblock is on ->fs_supers, ->s_umount is held exclusive,
    ->s_active is 1.  Along comes two more processes, trying to mount the same
    thing; sget() in each is picking that superblock, bumping ->s_count and
    trying to grab ->s_umount.  ->s_active is 3 now.  Original mount(2)
    finally gets to deactivate_locked_super() on failure; ->s_active is 2,
    superblock is still ->fs_supers because shutdown will *not* happen until
    ->s_active hits 0.  ->s_umount is dropped and now we have two processes
    chasing each other:
    s_active = 2, A acquired ->s_umount, B blocked
    A sees that the damn thing is stillborn, does deactivate_locked_super()
    s_active = 1, A drops ->s_umount, B gets it
    A restarts the search and finds the same superblock.  And bumps it ->s_active.
    s_active = 2, B holds ->s_umount, A blocked on trying to get it
    ... and we are in the earlier situation with A and B switched places.
    
    The root cause, of course, is that ->s_active should not grow until we'd
    got MS_BORN.  Then failing ->mount() will have deactivate_locked_super()
    shut the damn thing down.  Fortunately, it's easy to do - the key point
    is that grab_super() is called only for superblocks currently on ->fs_supers,
    so it can bump ->s_count and grab ->s_umount first, then check MS_BORN and
    bump ->s_active; we must never increment ->s_count for superblocks past
    ->kill_sb(), but grab_super() is never called for those.
    
    The bug is pretty old; we would've caught it by now, if not for accidental
    exclusion between sget() for block filesystems; the things like cgroup or
    e.g. mtd-based filesystems don't have anything of that sort, so they get
    bitten.  The right way to deal with that is obviously to fix sget()...
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    acfec9a5
super.c 34.6 KB