On Fri, Aug 13, 2021 at 02:19:11PM -0600, Jens Axboe wrote:
> On 8/13/21 2:17 PM, Keith Busch wrote:
> > On Thu, Aug 12, 2021 at 11:41:58AM -0600, Jens Axboe wrote:
> >> Indeed. Wonder if we should make that a small helper, as any clear of
> >> REQ_HIPRI should clear BIO_PERCPU_CACHE as well.
> >>
> >>
> >> diff --git a/block/blk-core.c b/block/blk-core.c
> >> index 7e852242f4cc..d2722ecd4d9b 100644
> >> --- a/block/blk-core.c
> >> +++ b/block/blk-core.c
> >> @@ -821,11 +821,8 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
> >> }
> >> }
> >>
> >> - if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags)) {
> >> - /* can't support alloc cache if we turn off polling */
> >> - bio_clear_flag(bio, BIO_PERCPU_CACHE);
> >> - bio->bi_opf &= ~REQ_HIPRI;
> >> - }
> >> + if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> >> + bio_clear_hipri(bio);
> >
> > Since BIO_PERCPU_CACHE doesn't work without REQ_HIRPI, should this check
> > look more like this?
> >
> > if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> > bio->bi_opf &= ~REQ_HIPRI;
> > if (!(bio->bi_opf & REQ_HIPRI))
> > bio_clear_flag(bio, BIO_PERCPU_CACHE);
> >
> > I realise the only BIO_PERCPU_CACHE user in this series never sets it
> > without REQ_HIPRI, but it looks like a problem waiting to happen if
> > nothing enforces this pairing: someone could set the CACHE flag on a
> > QUEUE_FLAG_POLL enabled queue without setting HIPRI and get the wrong
> > bio_put() action.
>
> I'd rather turn that into a WARN_ON or similar. But probably better to
> do that on the freeing side, honestly. That'll be the most reliable way,
> but a shame to add cycles to the hot path...
Yeah, it is a coding error if that happened, so a WARN sounds okay. I
also don't like adding these kinds of checks, so please feel free to not
include it if you think the usage is clear enough.
