Re: [PATCH 01/12] blkcg: fix ref count issue with bio_blkcg using task_css

[Tejun Heo]

On Thu, Sep 06, 2018 at 05:10:34PM -0400, Dennis Zhou wrote:
> From: "Dennis Zhou (Facebook)" <dennisszhou@xxxxxxxxx>
> 
> 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: Dennis Zhou <dennisszhou@xxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

-- 
tejun