Re: [PATCH AUTOSEL 4.20 49/77] block: cover another queue enter recursion via BIO_QUEUE_ENTERED

[Ming Lei]

On Thu, Feb 14, 2019 at 09:08:27PM -0500, Sasha Levin wrote:
> From: Ming Lei <ming.lei@xxxxxxxxxx>
> 
> [ Upstream commit 698cef173983b086977e633e46476e0f925ca01e ]
> 
> Except for blk_queue_split(), bio_split() is used for splitting bio too,
> then the remained bio is often resubmit to queue via generic_make_request().
> So the same queue enter recursion exits in this case too. Unfortunatley
> commit cd4a4ae4683dc2 doesn't help this case.
> 
> This patch covers the above case by setting BIO_QUEUE_ENTERED before calling
> q->make_request_fn.
> 
> In theory the per-bio flag is used to simulate one stack variable, it is
> just fine to clear it after q->make_request_fn is returned. Especially
> the same bio can't be submitted from another context.
> 
> Fixes: cd4a4ae4683dc2 ("block: don't use blocking queue entered for recursive bio submits")
> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: NeilBrown <neilb@xxxxxxxx>
> Reviewed-by:  Mike Snitzer <snitzer@xxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  block/blk-core.c  | 11 +++++++++++
>  block/blk-merge.c | 10 ----------
>  2 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index deb56932f8c4..22260339f66f 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2449,7 +2449,18 @@ blk_qc_t generic_make_request(struct bio *bio)
>  			/* Create a fresh bio_list for all subordinate requests */
>  			bio_list_on_stack[1] = bio_list_on_stack[0];
>  			bio_list_init(&bio_list_on_stack[0]);
> +
> +			/*
> +			 * Since we're recursing into make_request here, ensure
> +			 * that we mark this bio as already having entered the queue.
> +			 * If not, and the queue is going away, we can get stuck
> +			 * forever on waiting for the queue reference to drop. But
> +			 * that will never happen, as we're already holding a
> +			 * reference to it.
> +			 */
> +			bio_set_flag(bio, BIO_QUEUE_ENTERED);
>  			ret = q->make_request_fn(q, bio);
> +			bio_clear_flag(bio, BIO_QUEUE_ENTERED);
>  
>  			/* sort new bios into those for a lower level
>  			 * and those for the same level
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 7695034f4b87..adfe835ab258 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -272,16 +272,6 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)
>  		/* there isn't chance to merge the splitted bio */
>  		split->bi_opf |= REQ_NOMERGE;
>  
> -		/*
> -		 * Since we're recursing into make_request here, ensure
> -		 * that we mark this bio as already having entered the queue.
> -		 * If not, and the queue is going away, we can get stuck
> -		 * forever on waiting for the queue reference to drop. But
> -		 * that will never happen, as we're already holding a
> -		 * reference to it.
> -		 */
> -		bio_set_flag(*bio, BIO_QUEUE_ENTERED);
> -
>  		bio_chain(split, *bio);
>  		trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
>  		generic_make_request(*bio);
> -- 
> 2.19.1
> 

This one should be dropped, given it has been reverted.

-- 
Ming