Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for tagset in elevator_exit()

[John Garry]

On 16/03/2021 17:00, Bart Van Assche wrote:
> On 3/16/21 9:15 AM, John Garry wrote:
> > I'll have a look at this ASAP -  a bit busy.
> >
> > But a quick scan and I notice this:
> >
> >  > @@ -226,6 +226,7 @@ static inline void 
> > __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx,
> >  >                          struct request *rq)
> >  >   {
> >  >       blk_mq_put_tag(hctx->tags, rq->mq_ctx, rq->tag);
> >  > +    rcu_assign_pointer(hctx->tags->rqs[rq->tag], NULL);
> >
> > Wasn't a requirement to not touch the fastpath at all, including even 
> > if only NULLifying a pointer?
> >
> > IIRC, Kashyap some time ago had a patch like above (but without RCU 
> > usage), but the request from Jens was to not touch the fastpath.
> >
> > Maybe I'm mistaken - I will try to dig up the thread.

Hi Bart,

> I agree that Jens asked at the end of 2018 not to touch the fast path to 
> fix this use-after-free (maybe that request has been repeated more 
> recently). If Jens or anyone else feels strongly about not clearing 
> hctx->tags->rqs[rq->tag] from the fast path then I will make that 
> change. 

Is that possible for this same approach? I need to check the code more..

And don't we still have the problem that some iter callbacks may 
sleep/block, which is not allowed in an RCU read-side critical section?

> My motivation for clearing these pointers from the fast path is 
> as follows:
> - This results in code that is easier to read and easier to maintain.
> - Every modern CPU pipelines store instructions so the performance 
> impact of adding an additional store should be small.
> - Since the block layer has a tendency to reuse tags that have been 
> freed recently, it is likely that hctx->tags->rqs[rq->tag] will be used 
> for a next request and hence that it will have to be loaded into the CPU 
> cache anyway.

Those points make sense to me, but obviously it's the maintainers call.

Thanks,
john