Re: [PATCH 1/2] blk-mq: fixup request re-insert in blk_mq_try_issue_list_directly()

[Ming Lei <ming.lei@redhat.com>]

On Thu, Sep 19, 2019 at 11:45:46AM +0200, Hannes Reinecke wrote:
> From: Hannes Reinecke <hare@suse.com>
> 
> When blk_mq_request_issue_directly() returns BLK_STS_RESOURCE we
> need to requeue the I/O, but adding it to the global request list
> will mess up with the passed-in request list. So re-add the request

We always add request to hctx->dispatch_list after .queue_rq() returns
BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE, so what is the messing up?

> to the original list and leave it to the caller to handle situations
> where the list wasn't completely emptied.
> 
> Signed-off-by: Hannes Reinecke <hare@suse.com>
> ---
>  block/blk-mq.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index b038ec680e84..44ff3c1442a4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1899,8 +1899,7 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
>  		if (ret != BLK_STS_OK) {
>  			if (ret == BLK_STS_RESOURCE ||
>  					ret == BLK_STS_DEV_RESOURCE) {
> -				blk_mq_request_bypass_insert(rq,
> -							list_empty(list));
> +				list_add(list, &rq->queuelist);

This way may let this request(DONTPREP set) to be merged with other rq
or bio, and potential data corruption may be caused, please see commit:

	c616cbee97ae blk-mq: punt failed direct issue to dispatch list


Thanks,
Ming