On Mon, Nov 26, 2018 at 04:19:34PM -0500, Dennis Zhou wrote: > The bio_blkcg() function turns out to be inconsistent and consequently > dangerous to use. The first part returns a blkcg where a reference is > owned by the bio meaning it does not need to be rcu protected. However, > the third case, the last line, is problematic: > > return css_to_blkcg(task_css(current, io_cgrp_id)); > > This can race against task migration and the cgroup dying. It is also > semantically different as it must be called rcu protected and is > susceptible to failure when trying to get a reference to it. > > 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(). In blk-iolatency, association is > moved above the bio_blkcg() call to ensure it will not return %NULL. > > BFQ uses the old bio_blkcg() function, but I do not want to address it > in this series due to the complexity. I have created a private version > documenting the inconsistency and noting not to use it. > > Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx> > Acked-by: Tejun Heo <tj@xxxxxxxxxx> Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx> Thanks, Josef
