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

[Sasha Levin <sashal@kernel.org>]

On Fri, Feb 15, 2019 at 10:28:08AM +0800, Ming Lei wrote:
>On Thu, Feb 14, 2019 at 09:08:27PM -0500, Sasha Levin wrote:
>> From: Ming Lei <ming.lei@redhat.com>
>>
>> [ 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@I-love.SAKURA.ne.jp>
>> Cc: NeilBrown <neilb@suse.com>
>> Reviewed-by:  Mike Snitzer <snitzer@redhat.com>
>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> Signed-off-by: Sasha Levin <sashal@kernel.org>
>> ---
>>  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.

Dropped, thank you.

--
Thanks,
Sasha