blk-mq: fix sysfs registration/unregistration race

There is a race between cpu hotplug handling and adding/deleting
gendisk for blk-mq, where both are trying to register and unregister
the same sysfs entries.

null_add_dev
    --> blk_mq_init_queue
        --> blk_mq_init_allocated_queue
            --> add to 'all_q_list' (*)
    --> add_disk
        --> blk_register_queue
            --> blk_mq_register_disk (++)

null_del_dev
    --> del_gendisk
        --> blk_unregister_queue
            --> blk_mq_unregister_disk (--)
    --> blk_cleanup_queue
        --> blk_mq_free_queue
            --> del from 'all_q_list' (*)

blk_mq_queue_reinit
    --> blk_mq_sysfs_unregister (-)
    --> blk_mq_sysfs_register (+)

While the request queue is added to 'all_q_list' (*),
blk_mq_queue_reinit() can be called for the queue anytime by CPU
hotplug callback.  But blk_mq_sysfs_unregister (-) and
blk_mq_sysfs_register (+) in blk_mq_queue_reinit must not be called
before blk_mq_register_disk (++) and after blk_mq_unregister_disk (--)
is finished.  Because '/sys/block/*/mq/' is not exists.

There has already been BLK_MQ_F_SYSFS_UP flag in hctx->flags which can
be used to track these sysfs stuff, but it is only fixing this issue
partially.

In order to fix it completely, we just need per-queue flag instead of
per-hctx flag with appropriate locking.  So this introduces
q->mq_sysfs_init_done which is properly protected with all_q_mutex.

Also, we need to ensure that blk_mq_map_swqueue() is called with
all_q_mutex is held.  Since hctx->nr_ctx is reset temporarily and
updated in blk_mq_map_swqueue(), so we should avoid
blk_mq_register_hctx() seeing the temporary hctx->nr_ctx value
in CPU hotplug handling or adding/deleting gendisk .

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Cc: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 279c5d674edf3cb38627feb360eb745194eecd4e..189f5ae6522af50093d3e4225bd05df31b7edc87 100644 (file)
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -343,7 +343,7 @@ static void blk_mq_unregister_hctx(struct blk_mq_hw_ctx *hctx)
        struct blk_mq_ctx *ctx;
        int i;
 
-       if (!hctx->nr_ctx || !(hctx->flags & BLK_MQ_F_SYSFS_UP))
+       if (!hctx->nr_ctx)
                return;
 
        hctx_for_each_ctx(hctx, ctx, i)
@@ -358,7 +358,7 @@ static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx)
        struct blk_mq_ctx *ctx;
        int i, ret;
 
-       if (!hctx->nr_ctx || !(hctx->flags & BLK_MQ_F_SYSFS_UP))
+       if (!hctx->nr_ctx)
                return 0;
 
        ret = kobject_add(&hctx->kobj, &q->mq_kobj, "%u", hctx->queue_num);
@@ -381,6 +381,8 @@ void blk_mq_unregister_disk(struct gendisk *disk)
        struct blk_mq_ctx *ctx;
        int i, j;
 
+       blk_mq_disable_hotplug();
+
        queue_for_each_hw_ctx(q, hctx, i) {
                blk_mq_unregister_hctx(hctx);
 
@@ -395,6 +397,9 @@ void blk_mq_unregister_disk(struct gendisk *disk)
        kobject_put(&q->mq_kobj);
 
        kobject_put(&disk_to_dev(disk)->kobj);
+
+       q->mq_sysfs_init_done = false;
+       blk_mq_enable_hotplug();
 }
 
 static void blk_mq_sysfs_init(struct request_queue *q)
@@ -425,27 +430,30 @@ int blk_mq_register_disk(struct gendisk *disk)
        struct blk_mq_hw_ctx *hctx;
        int ret, i;
 
+       blk_mq_disable_hotplug();
+
        blk_mq_sysfs_init(q);
 
        ret = kobject_add(&q->mq_kobj, kobject_get(&dev->kobj), "%s", "mq");
        if (ret < 0)
-               return ret;
+               goto out;
 
        kobject_uevent(&q->mq_kobj, KOBJ_ADD);
 
        queue_for_each_hw_ctx(q, hctx, i) {
-               hctx->flags |= BLK_MQ_F_SYSFS_UP;
                ret = blk_mq_register_hctx(hctx);
                if (ret)
                        break;
        }
 
-       if (ret) {
+       if (ret)
                blk_mq_unregister_disk(disk);
-               return ret;
-       }
+       else
+               q->mq_sysfs_init_done = true;
+out:
+       blk_mq_enable_hotplug();
 
-       return 0;
+       return ret;
 }
 EXPORT_SYMBOL_GPL(blk_mq_register_disk);
 
@@ -454,6 +462,9 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
        struct blk_mq_hw_ctx *hctx;
        int i;
 
+       if (!q->mq_sysfs_init_done)
+               return;
+
        queue_for_each_hw_ctx(q, hctx, i)
                blk_mq_unregister_hctx(hctx);
 }
@@ -463,6 +474,9 @@ int blk_mq_sysfs_register(struct request_queue *q)
        struct blk_mq_hw_ctx *hctx;
        int i, ret = 0;
 
+       if (!q->mq_sysfs_init_done)
+               return ret;
+
        queue_for_each_hw_ctx(q, hctx, i) {
                ret = blk_mq_register_hctx(hctx);
                if (ret)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2fd7283ec62ba29bd176e2d7f3149f5aa228d1ea..0262131ac5f24b4f2618e49229176cb45c9f90ac 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2035,13 +2035,13 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
                goto err_hctxs;
 
        mutex_lock(&all_q_mutex);
-       list_add_tail(&q->all_q_node, &all_q_list);
-       mutex_unlock(&all_q_mutex);
 
+       list_add_tail(&q->all_q_node, &all_q_list);
        blk_mq_add_queue_tag_set(set, q);
-
        blk_mq_map_swqueue(q);
 
+       mutex_unlock(&all_q_mutex);
+
        return q;
 
 err_hctxs:

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 37d1602c4f7aa08b464577c675910046a4db3dde..b80ba4572a3149a33c4e7288271df0f66b22c242 100644 (file)
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -145,7 +145,6 @@ enum {
        BLK_MQ_F_SHOULD_MERGE   = 1 << 0,
        BLK_MQ_F_TAG_SHARED     = 1 << 1,
        BLK_MQ_F_SG_MERGE       = 1 << 2,
-       BLK_MQ_F_SYSFS_UP       = 1 << 3,
        BLK_MQ_F_DEFER_ISSUE    = 1 << 4,
        BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
        BLK_MQ_F_ALLOC_POLICY_BITS = 1,

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 99da9ebc73776af0a5efb69a73310f522b952b25..19c2e947d4d127364887a133d4b0d0ce92090e1c 100644 (file)
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -456,6 +456,8 @@ struct request_queue {
        struct blk_mq_tag_set   *tag_set;
        struct list_head        tag_set_list;
        struct bio_set          *bio_split;
+
+       bool                    mq_sysfs_init_done;
 };
 
 #define QUEUE_FLAG_QUEUED      1       /* uses generic tag queueing */