blk: Fix lock inversion between ioc lock and bfqd lock

Lockdep complains about lock inversion between ioc->lock and bfqd->lock:

bfqd -> ioc:
 put_io_context+0x33/0x90 -> ioc->lock grabbed
 blk_mq_free_request+0x51/0x140
 blk_put_request+0xe/0x10
 blk_attempt_req_merge+0x1d/0x30
 elv_attempt_insert_merge+0x56/0xa0
 blk_mq_sched_try_insert_merge+0x4b/0x60
 bfq_insert_requests+0x9e/0x18c0 -> bfqd->lock grabbed
 blk_mq_sched_insert_requests+0xd6/0x2b0
 blk_mq_flush_plug_list+0x154/0x280
 blk_finish_plug+0x40/0x60
 ext4_writepages+0x696/0x1320
 do_writepages+0x1c/0x80
 __filemap_fdatawrite_range+0xd7/0x120
 sync_file_range+0xac/0xf0

ioc->bfqd:
 bfq_exit_icq+0xa3/0xe0 -> bfqd->lock grabbed
 put_io_context_active+0x78/0xb0 -> ioc->lock grabbed
 exit_io_context+0x48/0x50
 do_exit+0x7e9/0xdd0
 do_group_exit+0x54/0xc0

To avoid this inversion we change blk_mq_sched_try_insert_merge() to not
free the merged request but rather leave that upto the caller similarly
to blk_mq_sched_try_merge(). And in bfq_insert_requests() we make sure
to free all the merged requests after dropping bfqd->lock.

Fixes: aee69d78dec0 ("block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210623093634.27879-3-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 9433d38..7279559 100644 (file)
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2345,9 +2345,9 @@ static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
 
        ret = blk_mq_sched_try_merge(q, bio, nr_segs, &free);
 
+       spin_unlock_irq(&bfqd->lock);
        if (free)
                blk_mq_free_request(free);
-       spin_unlock_irq(&bfqd->lock);
 
        return ret;
 }
@@ -5969,14 +5969,16 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
        struct bfq_queue *bfqq;
        bool idle_timer_disabled = false;
        unsigned int cmd_flags;
+       LIST_HEAD(free);
 
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
        if (!cgroup_subsys_on_dfl(io_cgrp_subsys) && rq->bio)
                bfqg_stats_update_legacy_io(q, rq);
 #endif
        spin_lock_irq(&bfqd->lock);
-       if (blk_mq_sched_try_insert_merge(q, rq)) {
+       if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
                spin_unlock_irq(&bfqd->lock);
+               blk_mq_free_requests(&free);
                return;
        }
 

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 4d97fb6..1398b52 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -846,18 +846,15 @@ static struct request *attempt_front_merge(struct request_queue *q,
        return NULL;
 }
 
-int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
-                         struct request *next)
+/*
+ * Try to merge 'next' into 'rq'. Return true if the merge happened, false
+ * otherwise. The caller is responsible for freeing 'next' if the merge
+ * happened.
+ */
+bool blk_attempt_req_merge(struct request_queue *q, struct request *rq,
+                          struct request *next)
 {
-       struct request *free;
-
-       free = attempt_merge(q, rq, next);
-       if (free) {
-               blk_put_request(free);
-               return 1;
-       }
-
-       return 0;
+       return attempt_merge(q, rq, next);
 }
 
 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 2403a5c..c838d81 100644 (file)
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -399,9 +399,10 @@ bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
        return ret;
 }
 
-bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
+bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq,
+                                  struct list_head *free)
 {
-       return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
+       return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq, free);
 }
 EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);
 

diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
index aff037c..5246ae0 100644 (file)
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -13,7 +13,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
                unsigned int nr_segs, struct request **merged_request);
 bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
                unsigned int nr_segs);
-bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq);
+bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq,
+                                  struct list_head *free);
 void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx);
 void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx);
 

diff --git a/block/blk-mq.h b/block/blk-mq.h
index 4b1ca7b..d08779f 100644 (file)
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -302,6 +302,17 @@ static inline struct blk_plug *blk_mq_plug(struct request_queue *q,
        return NULL;
 }
 
+/* Free all requests on the list */
+static inline void blk_mq_free_requests(struct list_head *list)
+{
+       while (!list_empty(list)) {
+               struct request *rq = list_entry_rq(list->next);
+
+               list_del_init(&rq->queuelist);
+               blk_mq_free_request(rq);
+       }
+}
+
 /*
  * For shared tag users, we track the number of currently active users
  * and attempt to provide a fair share of the tag depth for each of them.

diff --git a/block/blk.h b/block/blk.h
index 4fcd7a0..4b885c0 100644 (file)
--- a/block/blk.h
+++ b/block/blk.h
@@ -224,7 +224,7 @@ ssize_t part_timeout_store(struct device *, struct device_attribute *,
 void __blk_queue_split(struct bio **bio, unsigned int *nr_segs);
 int ll_back_merge_fn(struct request *req, struct bio *bio,
                unsigned int nr_segs);
-int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
+bool blk_attempt_req_merge(struct request_queue *q, struct request *rq,
                                struct request *next);
 unsigned int blk_recalc_rq_segments(struct request *rq);
 void blk_rq_set_mixed_merge(struct request *rq);

diff --git a/block/elevator.c b/block/elevator.c
index 85d0d4a..52ada14 100644 (file)
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -350,9 +350,11 @@ enum elv_merge elv_merge(struct request_queue *q, struct request **req,
  * we can append 'rq' to an existing request, so we can throw 'rq' away
  * afterwards.
  *
- * Returns true if we merged, false otherwise
+ * Returns true if we merged, false otherwise. 'free' will contain all
+ * requests that need to be freed.
  */
-bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
+bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq,
+                             struct list_head *free)
 {
        struct request *__rq;
        bool ret;
@@ -363,8 +365,10 @@ bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
        /*
         * First try one-hit cache.
         */
-       if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
+       if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq)) {
+               list_add(&rq->queuelist, free);
                return true;
+       }
 
        if (blk_queue_noxmerges(q))
                return false;
@@ -378,6 +382,7 @@ bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
                if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
                        break;
 
+               list_add(&rq->queuelist, free);
                /* The merged request could be merged with others, try again */
                ret = true;
                rq = __rq;

diff --git a/block/mq-deadline-main.c b/block/mq-deadline-main.c
index 4815e53..9db6da9 100644 (file)
--- a/block/mq-deadline-main.c
+++ b/block/mq-deadline-main.c
@@ -719,6 +719,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
        struct dd_per_prio *per_prio;
        enum dd_prio prio;
        struct dd_blkcg *blkcg;
+       LIST_HEAD(free);
 
        lockdep_assert_held(&dd->lock);
 
@@ -742,8 +743,10 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
        WARN_ON_ONCE(rq->elv.priv[0]);
        rq->elv.priv[0] = blkcg;
 
-       if (blk_mq_sched_try_insert_merge(q, rq))
+       if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
+               blk_mq_free_requests(&free);
                return;
+       }
 
        trace_block_rq_insert(rq);
 

diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 783ecb3..ef9ceea 100644 (file)
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -117,7 +117,8 @@ extern void elv_merge_requests(struct request_queue *, struct request *,
                               struct request *);
 extern void elv_merged_request(struct request_queue *, struct request *,
                enum elv_merge);
-extern bool elv_attempt_insert_merge(struct request_queue *, struct request *);
+extern bool elv_attempt_insert_merge(struct request_queue *, struct request *,
+                                    struct list_head *);
 extern struct request *elv_former_request(struct request_queue *, struct request *);
 extern struct request *elv_latter_request(struct request_queue *, struct request *);
 void elevator_init_mq(struct request_queue *q);