Re: [PATCH BUGFIX] block, bfq: access and cache blkg data only when safe

[Tejun Heo <tj@kernel.org>]

Hello, Paolo.

On Sat, May 20, 2017 at 09:27:33AM +0200, Paolo Valente wrote:
> Consider a process or a group that is moved from a given source group
> to a different group, or simply removed from a group (although I
> didn't yet succeed in just removing a process from a group :) ).  The
> pointer to the [b|c]fq_group contained in the schedulable entity
> belonging to the source group *is not* updated, in BFQ, if the entity
> is idle, and *is not* updated *unconditionally* in CFQ.  The update
> will happen in bfq_get_rq_private or cfq_set_request, on the arrival
> of a new request.  But, if the move happens right after the arrival of
> a request, then all the scheduler functions executed until a new
> request arrives for that entity will see a stale [b|c]fq_group.  Much

Limited staleness is fine.  Especially in this case, it isn't too
weird to claim that the order between the two operations isn't clearly
defined.

> worse, if also a blkcg_deactivate_policy or a blkg_destroy are
> executed right after the move, then both the policy data pointed by
> the [b|c]fq_group and the [b|c]fq_group itself may be deallocated.
> So, all the functions of the scheduler invoked before next request
> arrival may use dangling references!

Hmm... but cfq_group is allocated along with blkcg and blkcg always
ensures that there are no blkg left before freeing the pd area in
blkcg_css_offline().

Thanks.

-- 
tejun