blk-mq-debugfs: warn about possible deadlock

Creating new debugfs entries can trigger fs reclaim, hence we can't do
this with queue frozen, meanwhile, other locks that can be held while
queue is frozen should not be held as well.

Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 5c7cadf51a88f007dfc929da1105344f377ee67e..faeaa1fc86a717f6061636c48a9014ffb9592391 100644 (file)
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -608,9 +608,23 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_ctx_attrs[] = {
        {},
 };
 
-static void debugfs_create_files(struct dentry *parent, void *data,
+static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
+                                void *data,
                                 const struct blk_mq_debugfs_attr *attr)
 {
+       lockdep_assert_held(&q->debugfs_mutex);
+       /*
+        * Creating new debugfs entries with queue freezed has the risk of
+        * deadlock.
+        */
+       WARN_ON_ONCE(q->mq_freeze_depth != 0);
+       /*
+        * debugfs_mutex should not be nested under other locks that can be
+        * grabbed while queue is frozen.
+        */
+       lockdep_assert_not_held(&q->elevator_lock);
+       lockdep_assert_not_held(&q->rq_qos_mutex);
+
        if (IS_ERR_OR_NULL(parent))
                return;
 
@@ -624,7 +638,7 @@ void blk_mq_debugfs_register(struct request_queue *q)
        struct blk_mq_hw_ctx *hctx;
        unsigned long i;
 
-       debugfs_create_files(q->debugfs_dir, q, blk_mq_debugfs_queue_attrs);
+       debugfs_create_files(q, q->debugfs_dir, q, blk_mq_debugfs_queue_attrs);
 
        queue_for_each_hw_ctx(q, hctx, i) {
                if (!hctx->debugfs_dir)
@@ -643,7 +657,8 @@ static void blk_mq_debugfs_register_ctx(struct blk_mq_hw_ctx *hctx,
        snprintf(name, sizeof(name), "cpu%u", ctx->cpu);
        ctx_dir = debugfs_create_dir(name, hctx->debugfs_dir);
 
-       debugfs_create_files(ctx_dir, ctx, blk_mq_debugfs_ctx_attrs);
+       debugfs_create_files(hctx->queue, ctx_dir, ctx,
+                            blk_mq_debugfs_ctx_attrs);
 }
 
 void blk_mq_debugfs_register_hctx(struct request_queue *q,
@@ -659,7 +674,8 @@ void blk_mq_debugfs_register_hctx(struct request_queue *q,
        snprintf(name, sizeof(name), "hctx%u", hctx->queue_num);
        hctx->debugfs_dir = debugfs_create_dir(name, q->debugfs_dir);
 
-       debugfs_create_files(hctx->debugfs_dir, hctx, blk_mq_debugfs_hctx_attrs);
+       debugfs_create_files(q, hctx->debugfs_dir, hctx,
+                            blk_mq_debugfs_hctx_attrs);
 
        hctx_for_each_ctx(hctx, ctx, i)
                blk_mq_debugfs_register_ctx(hctx, ctx);
@@ -712,7 +728,7 @@ void blk_mq_debugfs_register_sched(struct request_queue *q)
 
        q->sched_debugfs_dir = debugfs_create_dir("sched", q->debugfs_dir);
 
-       debugfs_create_files(q->sched_debugfs_dir, q, e->queue_debugfs_attrs);
+       debugfs_create_files(q, q->sched_debugfs_dir, q, e->queue_debugfs_attrs);
 }
 
 void blk_mq_debugfs_unregister_sched(struct request_queue *q)
@@ -751,7 +767,8 @@ static void blk_mq_debugfs_register_rqos(struct rq_qos *rqos)
                                                         q->debugfs_dir);
 
        rqos->debugfs_dir = debugfs_create_dir(dir_name, q->rqos_debugfs_dir);
-       debugfs_create_files(rqos->debugfs_dir, rqos, rqos->ops->debugfs_attrs);
+       debugfs_create_files(q, rqos->debugfs_dir, rqos,
+                            rqos->ops->debugfs_attrs);
 }
 
 void blk_mq_debugfs_register_rq_qos(struct request_queue *q)
@@ -788,7 +805,7 @@ void blk_mq_debugfs_register_sched_hctx(struct request_queue *q,
 
        hctx->sched_debugfs_dir = debugfs_create_dir("sched",
                                                     hctx->debugfs_dir);
-       debugfs_create_files(hctx->sched_debugfs_dir, hctx,
+       debugfs_create_files(q, hctx->sched_debugfs_dir, hctx,
                             e->hctx_debugfs_attrs);
 }