Date:   Sun, 19 Apr 2020 19:45:23 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org
Cc:     mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: [PATCH v2 04/10] block: revert back to synchronous request_queue removal

Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
v4.12 moved the work behind blk_release_queue() into a workqueue after a
splat floated around which indicated some work on blk_release_queue()
could sleep in blk_exit_rl(). This splat would be possible when a driver
called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
as its final call) from an atomic context.

blk_put_queue() decrements the refcount for the request_queue
kobject, and upon reaching 0 blk_release_queue() is called. Although
blk_exit_rl() is now removed through commit db6d9952356 ("block: remove
request_list code"), we reserve the right to be able to sleep within
blk_release_queue() context. If you see no other way and *have* be
in atomic context when you driver calls the last blk_put_queue()
you can always just increase your block device's reference count with
bdgrab() as this can be done in atomic context and the request_queue
removal would be left to upper layers later. We document this bit of
tribal knowledge as well now, and adjust kdoc format a bit.

We revert back to synchronous request_queue removal because asynchronous
removal creates a regression with expected userspace interaction with
several drivers. An example is when removing the loopback driver and
issues ioctl from userspace to do so, upon return and if successful one
expects the device to be removed. Moving to asynchronous request_queue
removal could have broken many scripts which relied on the removal to
have been completed if there was no error.

Using asynchronous request_queue removal however has helped us find
other bugs, in the future we can test what could break with this
arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.

Cc: Bart Van Assche <bvanassche@....org>
Cc: Omar Sandoval <osandov@...com>
Cc: Hannes Reinecke <hare@...e.com>
Cc: Nicolai Stange <nstange@...e.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: yu kuai <yukuai3@...wei.com>
Reviewed-by: Ming Lei <ming.lei@...hat.com>
Suggested-by: Nicolai Stange <nstange@...e.de>
Fixes: dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
---
 block/blk-core.c       | 19 +++++++++++++++++++
 block/blk-sysfs.c      | 38 +++++++++++++++++---------------------
 include/linux/blkdev.h |  2 --
 3 files changed, 36 insertions(+), 23 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 5aaae7a1b338..19e24d7f40ef 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -301,6 +301,17 @@ void blk_clear_pm_only(struct request_queue *q)
 }
 EXPORT_SYMBOL_GPL(blk_clear_pm_only);
 
+/**
+ * blk_put_queue - decrement the request_queue refcount
+ *
+ * @q: the request_queue structure to decrement the refcount for
+ *
+ * Decrements the refcount to the request_queue kobject, when this reaches
+ * 0 we'll have blk_release_queue() called. You should avoid calling
+ * this function in atomic context but if you really have to ensure you
+ * first refcount the block device with bdgrab() / bdput() so that the
+ * last decrement happens in blk_cleanup_queue().
+ */
 void blk_put_queue(struct request_queue *q)
 {
 	kobject_put(&q->kobj);
@@ -328,13 +339,21 @@ EXPORT_SYMBOL_GPL(blk_set_queue_dying);
 
 /**
  * blk_cleanup_queue - shutdown a request queue
+ *
  * @q: request queue to shutdown
  *
  * Mark @q DYING, drain all pending requests, mark @q DEAD, destroy and
  * put it.  All future requests will be failed immediately with -ENODEV.
+ *
+ * You should not call this function in atomic context. If you need to
+ * refcount a request_queue in atomic context, instead refcount the
+ * block device with bdgrab() / bdput().
  */
 void blk_cleanup_queue(struct request_queue *q)
 {
+	/* cannot be called from atomic context */
+	might_sleep();
+
 	WARN_ON_ONCE(blk_queue_registered(q));
 
 	/* mark @q DYING, no new request or merges will be allowed afterwards */
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 7072f408e69a..dc7985b7e4c5 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -860,22 +860,27 @@ static void blk_exit_queue(struct request_queue *q)
 	bdi_put(q->backing_dev_info);
 }
 
-
 /**
- * __blk_release_queue - release a request queue
- * @work: pointer to the release_work member of the request queue to be released
+ * blk_release_queue - release a request queue
+ *
+ * This function is called as part of the process when a block device is being
+ * unregistered. Releasing a request queue starts with blk_cleanup_queue(),
+ * which set the appropriate flags and then calls blk_put_queue() as the last
+ * step. blk_put_queue() decrements the reference counter of the request queue
+ * and once the reference counter reaches zero, this function is called to
+ * release all allocated resources of the request queue.
  *
- * Description:
- *     This function is called when a block device is being unregistered. The
- *     process of releasing a request queue starts with blk_cleanup_queue, which
- *     set the appropriate flags and then calls blk_put_queue, that decrements
- *     the reference counter of the request queue. Once the reference counter
- *     of the request queue reaches zero, blk_release_queue is called to release
- *     all allocated resources of the request queue.
+ * This function can sleep, and so we must ensure that the very last
+ * blk_put_queue() is never called from atomic context.
+ *
+ * @kobj: pointer to a kobject, who's container is a request_queue
  */
-static void __blk_release_queue(struct work_struct *work)
+static void blk_release_queue(struct kobject *kobj)
 {
-	struct request_queue *q = container_of(work, typeof(*q), release_work);
+	struct request_queue *q =
+		container_of(kobj, struct request_queue, kobj);
+
+	might_sleep();
 
 	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
 		blk_stat_remove_callback(q, q->poll_cb);
@@ -905,15 +910,6 @@ static void __blk_release_queue(struct work_struct *work)
 	call_rcu(&q->rcu_head, blk_free_queue_rcu);
 }
 
-static void blk_release_queue(struct kobject *kobj)
-{
-	struct request_queue *q =
-		container_of(kobj, struct request_queue, kobj);
-
-	INIT_WORK(&q->release_work, __blk_release_queue);
-	schedule_work(&q->release_work);
-}
-
 static const struct sysfs_ops queue_sysfs_ops = {
 	.show	= queue_attr_show,
 	.store	= queue_attr_store,
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index cc43c8e6516c..81f7ddb1587e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -582,8 +582,6 @@ struct request_queue {
 
 	size_t			cmd_size;
 
-	struct work_struct	release_work;
-
 #define BLK_MAX_WRITE_HINTS	5
 	u64			write_hints[BLK_MAX_WRITE_HINTS];
 };
-- 
2.25.1

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