| Message ID | 1486051573-13445-3-git-send-email-axboe@fb.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu, Feb 02, 2017 at 09:06:13AM -0700, Jens Axboe wrote: > If we end up doing a request-to-request merge when we have completed > a bio-to-request merge, we free the request from deep down in that > path. For blk-mq-sched, the merge path has to hold the appropriate > lock, but we don't need it for freeing the request. And in fact > holding the lock is problematic, since we are now calling the > mq sched put_rq_private() hook with the lock held. Other call paths > do not hold this lock. > > Fix this inconsistency by ensuring that the caller frees a merged > request. Then we can do it outside of the lock, making it both more > efficient and fixing the blk-mq-sched problem of invoking parts of > the scheduler with an unknown lock state. > > Reported-by: Paolo Valente <paolo.valente@linaro.org> Reviewed-by: Omar Sandoval <osandov@fb.com> > Signed-off-by: Jens Axboe <axboe@fb.com> > --- > block/blk-core.c | 12 +++++++++--- > block/blk-merge.c | 15 ++++++++++++--- > block/blk-mq-sched.c | 9 ++++++--- > block/blk-mq-sched.h | 3 ++- > block/mq-deadline.c | 8 ++++++-- > 5 files changed, 35 insertions(+), 12 deletions(-)
> Il giorno 02 feb 2017, alle ore 17:06, Jens Axboe <axboe@fb.com> ha scritto: > > If we end up doing a request-to-request merge when we have completed > a bio-to-request merge, we free the request from deep down in that > path. For blk-mq-sched, the merge path has to hold the appropriate > lock, but we don't need it for freeing the request. And in fact > holding the lock is problematic, since we are now calling the > mq sched put_rq_private() hook with the lock held. Other call paths > do not hold this lock. > > Fix this inconsistency by ensuring that the caller frees a merged > request. Then we can do it outside of the lock, making it both more > efficient and fixing the blk-mq-sched problem of invoking parts of > the scheduler with an unknown lock state. > > Reported-by: Paolo Valente <paolo.valente@linaro.org> > Signed-off-by: Jens Axboe <axboe@fb.com> > --- > block/blk-core.c | 12 +++++++++--- > block/blk-merge.c | 15 ++++++++++++--- > block/blk-mq-sched.c | 9 ++++++--- > block/blk-mq-sched.h | 3 ++- > block/mq-deadline.c | 8 ++++++-- > 5 files changed, 35 insertions(+), 12 deletions(-) > > diff --git a/block/blk-core.c b/block/blk-core.c > index a5726e01f839..00c90f8cd682 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -1591,7 +1591,7 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio) > { > struct blk_plug *plug; > int el_ret, where = ELEVATOR_INSERT_SORT; > - struct request *req; > + struct request *req, *free; > unsigned int request_count = 0; > unsigned int wb_acct; > > @@ -1632,15 +1632,21 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio) > if (el_ret == ELEVATOR_BACK_MERGE) { > if (bio_attempt_back_merge(q, req, bio)) { > elv_bio_merged(q, req, bio); > - if (!attempt_back_merge(q, req)) > + free = attempt_back_merge(q, req); > + if (!free) > elv_merged_request(q, req, el_ret); > + else > + __blk_put_request(q, free); > goto out_unlock; > } > } else if (el_ret == ELEVATOR_FRONT_MERGE) { > if (bio_attempt_front_merge(q, req, bio)) { > elv_bio_merged(q, req, bio); > - if (!attempt_front_merge(q, req)) > + free = attempt_front_merge(q, req); > + if (!free) > elv_merged_request(q, req, el_ret); > + else > + __blk_put_request(q, free); > goto out_unlock; > } > } > diff --git a/block/blk-merge.c b/block/blk-merge.c > index 3826fc32b72c..a373416dbc9a 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -733,9 +733,11 @@ static struct request *attempt_merge(struct request_queue *q, > if (blk_rq_cpu_valid(next)) > req->cpu = next->cpu; > > - /* owner-ship of bio passed from next to req */ > + /* > + * ownership of bio passed from next to req, return 'next' for > + * the caller to free > + */ > next->bio = NULL; > - __blk_put_request(q, next); > return next; > } > > @@ -763,12 +765,19 @@ int blk_attempt_req_merge(struct request_queue *q, struct request *rq, > struct request *next) > { > struct elevator_queue *e = q->elevator; > + struct request *free; > > if (!e->uses_mq && e->type->ops.sq.elevator_allow_rq_merge_fn) > if (!e->type->ops.sq.elevator_allow_rq_merge_fn(q, rq, next)) > return 0; > > - return attempt_merge(q, rq, next) != NULL; > + free = attempt_merge(q, rq, next); > + if (free) { > + __blk_put_request(q, free); > + return 1; > + } > + > + return 0; > } > Hi Jens, blk_attempt_req_merge is reached also from blk_mq_sched_try_insert_merge, which is invoked in mq-deadline. According to the current usage of the scheduler lock in mq-deadline, that lock should be taken before invoking blk_mq_sched_try_insert_merge too, as I pointed out in a previous reply of mine. But, if it is taken, then we have again the double-lock-state problem in put_rq_private. Unfortunately, from your general explanation on how to best protect shared data structures, I did not understand (sorry) whether the scheduler lock must or must not be taken around blk_mq_sched_try_insert_merge in the current implementation of mq-deadline. Could you patiently provide a thickheaded-proof, yes/no answer, if possible? Thanks, Paolo > bool blk_rq_merge_ok(struct request *rq, struct bio *bio) > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > index 114814ec3d49..d93b56d53c4e 100644 > --- a/block/blk-mq-sched.c > +++ b/block/blk-mq-sched.c > @@ -234,7 +234,8 @@ void blk_mq_sched_move_to_dispatch(struct blk_mq_hw_ctx *hctx, > } > EXPORT_SYMBOL_GPL(blk_mq_sched_move_to_dispatch); > > -bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio) > +bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, > + struct request **merged_request) > { > struct request *rq; > int ret; > @@ -244,7 +245,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio) > if (!blk_mq_sched_allow_merge(q, rq, bio)) > return false; > if (bio_attempt_back_merge(q, rq, bio)) { > - if (!attempt_back_merge(q, rq)) > + *merged_request = attempt_back_merge(q, rq); > + if (!*merged_request) > elv_merged_request(q, rq, ret); > return true; > } > @@ -252,7 +254,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio) > if (!blk_mq_sched_allow_merge(q, rq, bio)) > return false; > if (bio_attempt_front_merge(q, rq, bio)) { > - if (!attempt_front_merge(q, rq)) > + *merged_request = attempt_front_merge(q, rq); > + if (!*merged_request) > elv_merged_request(q, rq, ret); > return true; > } > diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h > index 9478aaeb48c5..3643686a54b8 100644 > --- a/block/blk-mq-sched.h > +++ b/block/blk-mq-sched.h > @@ -16,7 +16,8 @@ void blk_mq_sched_put_request(struct request *rq); > > void blk_mq_sched_request_inserted(struct request *rq); > bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx, struct request *rq); > -bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio); > +bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, > + struct request **merged_request); > bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio); > bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq); > void blk_mq_sched_restart_queues(struct blk_mq_hw_ctx *hctx); > diff --git a/block/mq-deadline.c b/block/mq-deadline.c > index 49583536698c..682fa64f55ff 100644 > --- a/block/mq-deadline.c > +++ b/block/mq-deadline.c > @@ -371,12 +371,16 @@ static bool dd_bio_merge(struct blk_mq_hw_ctx *hctx, struct bio *bio) > { > struct request_queue *q = hctx->queue; > struct deadline_data *dd = q->elevator->elevator_data; > - int ret; > + struct request *free = NULL; > + bool ret; > > spin_lock(&dd->lock); > - ret = blk_mq_sched_try_merge(q, bio); > + ret = blk_mq_sched_try_merge(q, bio, &free); > spin_unlock(&dd->lock); > > + if (free) > + blk_mq_free_request(free); > + > return ret; > } > > -- > 2.7.4 >
diff --git a/block/blk-core.c b/block/blk-core.c index a5726e01f839..00c90f8cd682 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1591,7 +1591,7 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio) { struct blk_plug *plug; int el_ret, where = ELEVATOR_INSERT_SORT; - struct request *req; + struct request *req, *free; unsigned int request_count = 0; unsigned int wb_acct; @@ -1632,15 +1632,21 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio) if (el_ret == ELEVATOR_BACK_MERGE) { if (bio_attempt_back_merge(q, req, bio)) { elv_bio_merged(q, req, bio); - if (!attempt_back_merge(q, req)) + free = attempt_back_merge(q, req); + if (!free) elv_merged_request(q, req, el_ret); + else + __blk_put_request(q, free); goto out_unlock; } } else if (el_ret == ELEVATOR_FRONT_MERGE) { if (bio_attempt_front_merge(q, req, bio)) { elv_bio_merged(q, req, bio); - if (!attempt_front_merge(q, req)) + free = attempt_front_merge(q, req); + if (!free) elv_merged_request(q, req, el_ret); + else + __blk_put_request(q, free); goto out_unlock; } } diff --git a/block/blk-merge.c b/block/blk-merge.c index 3826fc32b72c..a373416dbc9a 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -733,9 +733,11 @@ static struct request *attempt_merge(struct request_queue *q, if (blk_rq_cpu_valid(next)) req->cpu = next->cpu; - /* owner-ship of bio passed from next to req */ + /* + * ownership of bio passed from next to req, return 'next' for + * the caller to free + */ next->bio = NULL; - __blk_put_request(q, next); return next; } @@ -763,12 +765,19 @@ int blk_attempt_req_merge(struct request_queue *q, struct request *rq, struct request *next) { struct elevator_queue *e = q->elevator; + struct request *free; if (!e->uses_mq && e->type->ops.sq.elevator_allow_rq_merge_fn) if (!e->type->ops.sq.elevator_allow_rq_merge_fn(q, rq, next)) return 0; - return attempt_merge(q, rq, next) != NULL; + free = attempt_merge(q, rq, next); + if (free) { + __blk_put_request(q, free); + return 1; + } + + return 0; } bool blk_rq_merge_ok(struct request *rq, struct bio *bio) diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 114814ec3d49..d93b56d53c4e 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -234,7 +234,8 @@ void blk_mq_sched_move_to_dispatch(struct blk_mq_hw_ctx *hctx, } EXPORT_SYMBOL_GPL(blk_mq_sched_move_to_dispatch); -bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio) +bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, + struct request **merged_request) { struct request *rq; int ret; @@ -244,7 +245,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio) if (!blk_mq_sched_allow_merge(q, rq, bio)) return false; if (bio_attempt_back_merge(q, rq, bio)) { - if (!attempt_back_merge(q, rq)) + *merged_request = attempt_back_merge(q, rq); + if (!*merged_request) elv_merged_request(q, rq, ret); return true; } @@ -252,7 +254,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio) if (!blk_mq_sched_allow_merge(q, rq, bio)) return false; if (bio_attempt_front_merge(q, rq, bio)) { - if (!attempt_front_merge(q, rq)) + *merged_request = attempt_front_merge(q, rq); + if (!*merged_request) elv_merged_request(q, rq, ret); return true; } diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h index 9478aaeb48c5..3643686a54b8 100644 --- a/block/blk-mq-sched.h +++ b/block/blk-mq-sched.h @@ -16,7 +16,8 @@ void blk_mq_sched_put_request(struct request *rq); void blk_mq_sched_request_inserted(struct request *rq); bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx, struct request *rq); -bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio); +bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, + struct request **merged_request); bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio); bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq); void blk_mq_sched_restart_queues(struct blk_mq_hw_ctx *hctx); diff --git a/block/mq-deadline.c b/block/mq-deadline.c index 49583536698c..682fa64f55ff 100644 --- a/block/mq-deadline.c +++ b/block/mq-deadline.c @@ -371,12 +371,16 @@ static bool dd_bio_merge(struct blk_mq_hw_ctx *hctx, struct bio *bio) { struct request_queue *q = hctx->queue; struct deadline_data *dd = q->elevator->elevator_data; - int ret; + struct request *free = NULL; + bool ret; spin_lock(&dd->lock); - ret = blk_mq_sched_try_merge(q, bio); + ret = blk_mq_sched_try_merge(q, bio, &free); spin_unlock(&dd->lock); + if (free) + blk_mq_free_request(free); + return ret; }
If we end up doing a request-to-request merge when we have completed a bio-to-request merge, we free the request from deep down in that path. For blk-mq-sched, the merge path has to hold the appropriate lock, but we don't need it for freeing the request. And in fact holding the lock is problematic, since we are now calling the mq sched put_rq_private() hook with the lock held. Other call paths do not hold this lock. Fix this inconsistency by ensuring that the caller frees a merged request. Then we can do it outside of the lock, making it both more efficient and fixing the blk-mq-sched problem of invoking parts of the scheduler with an unknown lock state. Reported-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@fb.com> --- block/blk-core.c | 12 +++++++++--- block/blk-merge.c | 15 ++++++++++++--- block/blk-mq-sched.c | 9 ++++++--- block/blk-mq-sched.h | 3 ++- block/mq-deadline.c | 8 ++++++-- 5 files changed, 35 insertions(+), 12 deletions(-)