On Tue, Sep 03, 2024 at 04:16:52PM +0800, Muchun Song wrote:
> Supposing the following scenario.
>
> CPU0 CPU1
>
> blk_mq_insert_request() 1) store blk_mq_unquiesce_queue()
> blk_mq_run_hw_queue() blk_queue_flag_clear(QUEUE_FLAG_QUIESCED) 3) store
> if (blk_queue_quiesced()) 2) load blk_mq_run_hw_queues()
> return blk_mq_run_hw_queue()
> blk_mq_sched_dispatch_requests() if (!blk_mq_hctx_has_pending()) 4) load
> return
>
> The full memory barrier should be inserted between 1) and 2), as well as
> between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is
> cleared or CPU1 sees dispatch list or setting of bitmap of software queue.
> Otherwise, either CPU will not re-run the hardware queue causing starvation.
>
> So the first solution is to 1) add a pair of memory barrier to fix the
> problem, another solution is to 2) use hctx->queue->queue_lock to synchronize
> QUEUE_FLAG_QUIESCED. Here, we chose 2) to fix it since memory barrier is not
> easy to be maintained.
>
> Fixes: f4560ffe8cec1 ("blk-mq: use QUEUE_FLAG_QUIESCED to quiesce queue")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Muchun Song <muchun.song@xxxxxxxxx>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
thanks,
Ming
