[PATCH] block: cover another queue enter recursion via BIO_QUEUE_ENTERED

[PATCH] block: cover another queue enter recursion via BIO_QUEUE_ENTERED

[Ming Lei]

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: Mike Snitzer <snitzer@redhat.com>
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 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 3c5f61ceeb67..1ccec27d20c3 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1083,7 +1083,18 @@ 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 b990853f6de7..8777e286bd3f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -339,16 +339,6 @@ 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);
-- 
2.9.5

Re: block: cover another queue enter recursion via BIO_QUEUE_ENTERED

[Mike Snitzer]

On Tue, Jan 22 2019 at  3:20am -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> 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: Mike Snitzer <snitzer@redhat.com>
> Cc: NeilBrown <neilb@suse.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

Looks good, I'll drop my DM patch that set/cleared BIO_QUEUE_ENTERED.

Jens, you OK with picking this up for 5.0-rc?

Reviewed-by:  Mike Snitzer <snitzer@redhat.com>

Re: block: cover another queue enter recursion via BIO_QUEUE_ENTERED

[Jens Axboe]

On 1/22/19 10:15 AM, Mike Snitzer wrote:
> On Tue, Jan 22 2019 at  3:20am -0500,
> Ming Lei <ming.lei@redhat.com> wrote:
> 
>> 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: Mike Snitzer <snitzer@redhat.com>
>> Cc: NeilBrown <neilb@suse.com>
>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> Looks good, I'll drop my DM patch that set/cleared BIO_QUEUE_ENTERED.
> 
> Jens, you OK with picking this up for 5.0-rc?
> 
> Reviewed-by:  Mike Snitzer <snitzer@redhat.com>

I am, thanks both of you for looking into this. I'm a bit behind on email
and patches from over the long weekend.

-- 
Jens Axboe