[PATCH v6 01/10] blk-mq: Register /queue/mq after having registered /queue

[Bart Van Assche]

A later patch in this series will modify blk_mq_debugfs_register()
such that it uses q->kobj.parent to determine the name of a
request queue. Hence make sure that that pointer is initialized
before blk_mq_debugfs_register() is called. To avoid lock inversion,
protect sysfs / debugfs registration with the queue sysfs_lock
instead of the global mutex all_q_mutex.

Signed-off-by: Bart Van Assche <bart.vanass...@sandisk.com>
Reviewed-by: Hannes Reinecke <h...@suse.com>
Reviewed-by: Omar Sandoval <osan...@fb.com>
---
 block/blk-mq-sysfs.c | 35 ++++++++++++++++++++++++++++-------
 block/blk-mq.h       |  1 +
 block/blk-sysfs.c    |  6 +++---
 3 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index d745ab81033a..a2dbb1a48e72 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -253,6 +253,8 @@ static void __blk_mq_unregister_dev(struct device *dev, 
struct request_queue *q)
        struct blk_mq_hw_ctx *hctx;
        int i;
 
+       lockdep_assert_held(&q->sysfs_lock);
+
        queue_for_each_hw_ctx(q, hctx, i)
                blk_mq_unregister_hctx(hctx);
 
@@ -267,9 +269,9 @@ static void __blk_mq_unregister_dev(struct device *dev, 
struct request_queue *q)
 
 void blk_mq_unregister_dev(struct device *dev, struct request_queue *q)
 {
-       blk_mq_disable_hotplug();
+       mutex_lock(&q->sysfs_lock);
        __blk_mq_unregister_dev(dev, q);
-       blk_mq_enable_hotplug();
+       mutex_unlock(&q->sysfs_lock);
 }
 
 void blk_mq_hctx_kobj_init(struct blk_mq_hw_ctx *hctx)
@@ -302,12 +304,13 @@ void blk_mq_sysfs_init(struct request_queue *q)
        }
 }
 
-int blk_mq_register_dev(struct device *dev, struct request_queue *q)
+int __blk_mq_register_dev(struct device *dev, struct request_queue *q)
 {
        struct blk_mq_hw_ctx *hctx;
        int ret, i;
 
-       blk_mq_disable_hotplug();
+       WARN_ON_ONCE(!q->kobj.parent);
+       lockdep_assert_held(&q->sysfs_lock);
 
        ret = kobject_add(&q->mq_kobj, kobject_get(&dev->kobj), "%s", "mq");
        if (ret < 0)
@@ -327,8 +330,18 @@ int blk_mq_register_dev(struct device *dev, struct 
request_queue *q)
                __blk_mq_unregister_dev(dev, q);
        else
                q->mq_sysfs_init_done = true;
+
 out:
-       blk_mq_enable_hotplug();
+       return ret;
+}
+
+int blk_mq_register_dev(struct device *dev, struct request_queue *q)
+{
+       int ret;
+
+       mutex_lock(&q->sysfs_lock);
+       ret = __blk_mq_register_dev(dev, q);
+       mutex_unlock(&q->sysfs_lock);
 
        return ret;
 }
@@ -339,13 +352,17 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
        struct blk_mq_hw_ctx *hctx;
        int i;
 
+       mutex_lock(&q->sysfs_lock);
        if (!q->mq_sysfs_init_done)
-               return;
+               goto unlock;
 
        blk_mq_debugfs_unregister_hctxs(q);
 
        queue_for_each_hw_ctx(q, hctx, i)
                blk_mq_unregister_hctx(hctx);
+
+unlock:
+       mutex_unlock(&q->sysfs_lock);
 }
 
 int blk_mq_sysfs_register(struct request_queue *q)
@@ -353,8 +370,9 @@ int blk_mq_sysfs_register(struct request_queue *q)
        struct blk_mq_hw_ctx *hctx;
        int i, ret = 0;
 
+       mutex_lock(&q->sysfs_lock);
        if (!q->mq_sysfs_init_done)
-               return ret;
+               goto unlock;
 
        blk_mq_debugfs_register_hctxs(q);
 
@@ -364,5 +382,8 @@ int blk_mq_sysfs_register(struct request_queue *q)
                        break;
        }
 
+unlock:
+       mutex_unlock(&q->sysfs_lock);
+
        return ret;
 }
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 524f44742816..7d955c756810 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -78,6 +78,7 @@ static inline struct blk_mq_hw_ctx *blk_mq_map_queue(struct 
request_queue *q,
  */
 extern void blk_mq_sysfs_init(struct request_queue *q);
 extern void blk_mq_sysfs_deinit(struct request_queue *q);
+extern int __blk_mq_register_dev(struct device *dev, struct request_queue *q);
 extern int blk_mq_sysfs_register(struct request_queue *q);
 extern void blk_mq_sysfs_unregister(struct request_queue *q);
 extern void blk_mq_hctx_kobj_init(struct blk_mq_hw_ctx *hctx);
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index f85723332288..3f37813ccbaf 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -877,9 +877,6 @@ int blk_register_queue(struct gendisk *disk)
        if (ret)
                return ret;
 
-       if (q->mq_ops)
-               blk_mq_register_dev(dev, q);
-
        /* Prevent changes through sysfs until registration is completed. */
        mutex_lock(&q->sysfs_lock);
 
@@ -889,6 +886,9 @@ int blk_register_queue(struct gendisk *disk)
                goto unlock;
        }
 
+       if (q->mq_ops)
+               __blk_mq_register_dev(dev, q);
+
        kobject_uevent(&q->kobj, KOBJ_ADD);
 
        wbt_enable_default(q);
-- 
2.12.2