block: free merged request in the caller

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>
Reviewed-by: Omar Sandoval <osandov@fb.com>

diff --git a/block/blk-core.c b/block/blk-core.c
index 76c42f559df8eaec96113c0b778bd6302fbd7e23..1d263311353ab2c836545a4fd4ba2d037eafa853 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1596,7 +1596,7 @@ 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;
 
@@ -1637,15 +1637,21 @@ 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 3826fc32b72cf811e8abf47ee16feab35075fd53..a373416dbc9a9282cd0a5b7e364bb4e340b79489 100644 (file)
--- 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 3ec52f4940944afa0ebb41cb87e35c1631008379..ee455e7cf9d8e492bb6a7c8ab36d45ebb9f5bb2a 100644 (file)
--- 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 add5f090a8cd6907e65f35889ad94bf3d7230a9b..5954859c86706cdb9eb6d95b8108cac5d0a36524 100644 (file)
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -15,7 +15,8 @@ struct request *blk_mq_sched_get_request(struct request_queue *q, struct bio *bi
 void blk_mq_sched_put_request(struct request *rq);
 
 void blk_mq_sched_request_inserted(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 8f91f21e866323095fbd252dec05e54f8127bfa9..d68d9c273a6657f4ac53549482da6b27bd072549 100644 (file)
--- 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;
 }