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

Damien Le Moal
2025-01-08T10:23:56Z

On 1/8/25 6:25 PM, 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>

Looks OK to me.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>

--
Damien Le Moal
Western Digital Research