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
