Re: [PATCH 03/10] block: don't update BLK_FEAT_POLL in __blk_mq_update_nr_hw_queues

Ming Lei
2025-01-08T10:31:37Z

On Wed, Jan 08, 2025 at 10:25:00AM +0100, Christoph Hellwig wrote:
> When __blk_mq_update_nr_hw_queues changes the number of tag sets, it
> might have to disable poll queues. Currently it does so by adjusting
> the BLK_FEAT_POLL, which is a bit against the intent of features that
> describe hardware / driver capabilities, but more importantly causes
> nasty lock order problems with the broadly held freeze when updating the
> number of hardware queues and the limits lock. Fix this by leaving
> BLK_FEAT_POLL alone, and instead check for the number of poll queues in
> the bio submission and poll handlers. While this adds extra work to the
> fast path, the variables are in cache lines used by these operations
> anyway, so it should be cheap enough.
>
> Fixes: 8023e144f9d6 ("block: move the poll flag to queue_limits")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
...

> /**
> * submit_bio_noacct - re-submit a bio to the block device layer for I/O
> * @bio: The bio describing the location in memory and on the device.
> @@ -805,8 +817,7 @@ void submit_bio_noacct(struct bio *bio)
> }
> }
>
> - if (!(q->limits.features & BLK_FEAT_POLL) &&
> - (bio->bi_opf & REQ_POLLED)) {
> + if ((bio->bi_opf & REQ_POLLED) && !bdev_can_poll(bdev)) {

submit_bio_noacct() is called without grabbing .q_usage_counter,
so tagset may be freed now, then use-after-free on q->tag_set?

Thanks,
Ming