From: Bart Van Assche <bvanassche@acm.org>
To: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org,
Christoph Hellwig <hch@infradead.org>,
Hannes Reinecke <hare@suse.com>,
Greg KH <gregkh@linuxfoundation.org>,
Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH V2 6/6] block: split .sysfs_lock into two locks
Date: Wed, 21 Aug 2019 09:18:08 -0700 [thread overview]
Message-ID: <6d97a960-52b5-5134-5382-dff73be00722@acm.org> (raw)
In-Reply-To: <20190821091506.21196-7-ming.lei@redhat.com>
On 8/21/19 2:15 AM, Ming Lei wrote:
> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> index 31bbf10d8149..a4cc40ddda86 100644
> --- a/block/blk-mq-sysfs.c
> +++ b/block/blk-mq-sysfs.c
> @@ -247,7 +247,7 @@ 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);
> + lockdep_assert_held(&q->sysfs_dir_lock);
>
> queue_for_each_hw_ctx(q, hctx, i)
> blk_mq_unregister_hctx(hctx);
> @@ -297,7 +297,7 @@ int __blk_mq_register_dev(struct device *dev, struct request_queue *q)
> int ret, i;
>
> WARN_ON_ONCE(!q->kobj.parent);
> - lockdep_assert_held(&q->sysfs_lock);
> + lockdep_assert_held(&q->sysfs_dir_lock);
>
> ret = kobject_add(q->mq_kobj, kobject_get(&dev->kobj), "%s", "mq");
> if (ret < 0)
blk_mq_unregister_dev and __blk_mq_register_dev() are only used by
blk_register_queue() and blk_unregister_queue(). It is the
responsibility of the callers of these function to serialize request
queue registration and unregistration. Is it really necessary to hold a
mutex around the blk_mq_unregister_dev and __blk_mq_register_dev()
calls? Or in other words, can it ever happen that multiple threads
invoke one or both functions concurrently?
> @@ -331,7 +331,7 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
> struct blk_mq_hw_ctx *hctx;
> int i;
>
> - mutex_lock(&q->sysfs_lock);
> + mutex_lock(&q->sysfs_dir_lock);
> if (!q->mq_sysfs_init_done)
> goto unlock;
>
> @@ -339,7 +339,7 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
> blk_mq_unregister_hctx(hctx);
>
> unlock:
> - mutex_unlock(&q->sysfs_lock);
> + mutex_unlock(&q->sysfs_dir_lock);
> }
>
> int blk_mq_sysfs_register(struct request_queue *q)
> @@ -347,7 +347,7 @@ int blk_mq_sysfs_register(struct request_queue *q)
> struct blk_mq_hw_ctx *hctx;
> int i, ret = 0;
>
> - mutex_lock(&q->sysfs_lock);
> + mutex_lock(&q->sysfs_dir_lock);
> if (!q->mq_sysfs_init_done)
> goto unlock;
>
> @@ -358,7 +358,7 @@ int blk_mq_sysfs_register(struct request_queue *q)
> }
>
> unlock:
> - mutex_unlock(&q->sysfs_lock);
> + mutex_unlock(&q->sysfs_dir_lock);
>
> return ret;
> }
blk_mq_sysfs_unregister() and blk_mq_sysfs_register() are only used by
__blk_mq_update_nr_hw_queues(). Calls to that function are serialized by
the tag_list_lock mutex. Is it really necessary to use any locking
inside these functions?
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 5b0b5224cfd4..5941a0176f87 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -938,6 +938,7 @@ int blk_register_queue(struct gendisk *disk)
> int ret;
> struct device *dev = disk_to_dev(disk);
> struct request_queue *q = disk->queue;
> + bool has_elevator = false;
>
> if (WARN_ON(!q))
> return -ENXIO;
> @@ -945,7 +946,6 @@ int blk_register_queue(struct gendisk *disk)
> WARN_ONCE(blk_queue_registered(q),
> "%s is registering an already registered queue\n",
> kobject_name(&dev->kobj));
> - blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
>
> /*
> * SCSI probing may synchronously create and destroy a lot of
> @@ -966,7 +966,7 @@ int blk_register_queue(struct gendisk *disk)
> return ret;
>
> /* Prevent changes through sysfs until registration is completed. */
> - mutex_lock(&q->sysfs_lock);
> + mutex_lock(&q->sysfs_dir_lock);
>
> ret = kobject_add(&q->kobj, kobject_get(&dev->kobj), "%s", "queue");
> if (ret < 0) {
> @@ -987,26 +987,37 @@ int blk_register_queue(struct gendisk *disk)
> blk_mq_debugfs_register(q);
> }
>
> - kobject_uevent(&q->kobj, KOBJ_ADD);
> -
> - wbt_enable_default(q);
> -
> - blk_throtl_register_queue(q);
> -
> + /*
> + * The queue's kobject ADD uevent isn't sent out, also the
> + * flag of QUEUE_FLAG_REGISTERED isn't set yet, so elevator
> + * switch won't happen at all.
> + */
> if (q->elevator) {
> - ret = elv_register_queue(q);
> + ret = elv_register_queue(q, false);
> if (ret) {
> - mutex_unlock(&q->sysfs_lock);
> - kobject_uevent(&q->kobj, KOBJ_REMOVE);
> + mutex_unlock(&q->sysfs_dir_lock);
> kobject_del(&q->kobj);
> blk_trace_remove_sysfs(dev);
> kobject_put(&dev->kobj);
> return ret;
> }
> + has_elevator = true;
> }
> +
> + mutex_lock(&q->sysfs_lock);
> + blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
> + wbt_enable_default(q);
> + blk_throtl_register_queue(q);
> + mutex_unlock(&q->sysfs_lock);
> +
> + /* Now everything is ready and send out KOBJ_ADD uevent */
> + kobject_uevent(&q->kobj, KOBJ_ADD);
> + if (has_elevator)
> + kobject_uevent(&q->elevator->kobj, KOBJ_ADD);
> +
> ret = 0;
> unlock:
> - mutex_unlock(&q->sysfs_lock);
> + mutex_unlock(&q->sysfs_dir_lock);
> return ret;
> }
My understanding is that the mutex_lock() / mutex_unlock() calls in this
function are necessary today to prevent concurrent changes of the
scheduler from this function and from sysfs. If the
kobject_uevent(KOBJ_ADD) call is moved, does that mean that all
mutex_lock() / mutex_unlock() calls can be left out from this function?
> EXPORT_SYMBOL_GPL(blk_register_queue);
> @@ -1021,6 +1032,7 @@ EXPORT_SYMBOL_GPL(blk_register_queue);
> void blk_unregister_queue(struct gendisk *disk)
> {
> struct request_queue *q = disk->queue;
> + bool has_elevator;
>
> if (WARN_ON(!q))
> return;
> @@ -1035,25 +1047,25 @@ void blk_unregister_queue(struct gendisk *disk)
> * concurrent elv_iosched_store() calls.
> */
> mutex_lock(&q->sysfs_lock);
> -
> blk_queue_flag_clear(QUEUE_FLAG_REGISTERED, q);
> + has_elevator = !!q->elevator;
> + mutex_unlock(&q->sysfs_lock);
>
> + mutex_lock(&q->sysfs_dir_lock);
> /*
> * Remove the sysfs attributes before unregistering the queue data
> * structures that can be modified through sysfs.
> */
> if (queue_is_mq(q))
> blk_mq_unregister_dev(disk_to_dev(disk), q);
> - mutex_unlock(&q->sysfs_lock);
>
> kobject_uevent(&q->kobj, KOBJ_REMOVE);
> kobject_del(&q->kobj);
> blk_trace_remove_sysfs(disk_to_dev(disk));
>
> - mutex_lock(&q->sysfs_lock);
> - if (q->elevator)
> + if (has_elevator)
> elv_unregister_queue(q);
> - mutex_unlock(&q->sysfs_lock);
> + mutex_unlock(&q->sysfs_dir_lock);
>
> kobject_put(&disk_to_dev(disk)->kobj);
> }
If this function would call kobject_del(&q->kobj) before doing anything
else, does that mean that all mutex_lock() / mutex_unlock() calls can be
left out from this function?
Thanks,
Bart.
next prev parent reply other threads:[~2019-08-21 16:18 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-21 9:15 [PATCH V2 0/6] block: don't acquire .sysfs_lock before removing mq & iosched kobjects Ming Lei
2019-08-21 9:15 ` [PATCH V2 1/6] block: Remove blk_mq_register_dev() Ming Lei
2019-08-21 9:15 ` [PATCH V2 2/6] block: don't hold q->sysfs_lock in elevator_init_mq Ming Lei
2019-08-21 15:51 ` Bart Van Assche
2019-08-21 9:15 ` [PATCH V2 3/6] blk-mq: don't hold q->sysfs_lock in blk_mq_map_swqueue Ming Lei
2019-08-21 15:53 ` Bart Van Assche
2019-08-26 2:11 ` Ming Lei
2019-08-21 9:15 ` [PATCH V2 4/6] blk-mq: don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs() Ming Lei
2019-08-21 15:56 ` Bart Van Assche
2019-08-26 2:25 ` Ming Lei
2019-08-21 9:15 ` [PATCH V2 5/6] block: add helper for checking if queue is registered Ming Lei
2019-08-21 15:57 ` Bart Van Assche
2019-08-21 9:15 ` [PATCH V2 6/6] block: split .sysfs_lock into two locks Ming Lei
2019-08-21 16:18 ` Bart Van Assche [this message]
2019-08-22 1:28 ` Ming Lei
2019-08-22 19:52 ` Bart Van Assche
2019-08-23 1:08 ` Ming Lei
2019-08-23 16:36 ` Bart Van Assche
2019-08-23 16:46 ` Bart Van Assche
2019-08-23 22:49 ` Ming Lei
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