[PATCH 04/11] block: don't update BLK_FEAT_POLL in __blk_mq_update_nr_hw_queues

[Ming Lei]

On Thu, Jan 09, 2025 at 06:57:25AM +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 at lst.de>
> Reviewed-by: Damien Le Moal <dlemoal at kernel.org>
> Reviewed-by: Johannes Thumshirn <johannes.thumshirn at wdc.com>

Reviewed-by: Ming Lei <ming.lei at redhat.com>

Thanks,
Ming