• Dennis Zhou (Facebook)'s avatar
    blkcg: fix ref count issue with bio_blkcg using task_css · 27e6fa99
    Dennis Zhou (Facebook) authored
    The accessor function bio_blkcg either returns the blkcg associated with
    the bio or finds one in the current context. This can cause an issue
    when trying to associate a bio with a blkcg. Particularly, it's the
    third case that is problematic:
    
    	return css_to_blkcg(task_css(current, io_cgrp_id));
    
    As the above may race against task migration and the cgroup exiting, it
    is not always ok to take a reference on the blkcg returned from
    bio_blkcg.
    
    This patch adds association ahead of calling bio_blkcg rather than
    after. This makes association a required and explicit step along the
    code paths for calling bio_blkcg. blk_get_rl is modified as well to get
    a reference to the blkcg it may use and blk_put_rl will always put the
    reference back. Association is also moved above the bio_blkcg call to
    ensure it will not return NULL in blk-iolatency.
    
    BFQ and CFQ utilize this flaw, but due to the complexity, I do not want
    to address this in this series. I've created a private version of the
    function with notes not to use it describing the flaw. Hopefully soon,
    that code can be cleaned up.
    Signed-off-by: default avatarDennis Zhou <dennisszhou@gmail.com>
    Acked-by: default avatarTejun Heo <tj@kernel.org>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    27e6fa99
bfq-cgroup.c 33.5 KB