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

Hannes Reinecke
2025-01-13T07:23:53Z

On 1/8/25 10:25, 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>
> ---
> block/blk-core.c | 17 ++++++++++++++---
> block/blk-mq.c | 17 +----------------
> block/blk-sysfs.c | 6 +++++-
> block/blk.h | 1 +
> 4 files changed, 21 insertions(+), 20 deletions(-)
>
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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