[PATCH v2 1/2] block: Add post_release() operation

[Bart Van Assche <bvanassche@acm.org>]

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

Add post_release() block device operation which provides a hook for
performing synchronous cleanup without disk->open_mutex held, which is
needed by the loop devices.

Real-world container engines, test suites, and system utilities rely on
fput() from __loop_clr_fd() being completed when lo_release() returns.
But changes which went to the v7.1 merge window broke an assumption that
there is no outstanding I/O when __loop_clr_fd() is called, causing NULL
pointer dereference problem in lo_rw_aio().

In order to fix this regression, we want to allow __loop_clr_fd() to flush
outstanding I/O. But calling drain_workqueue() from __loop_clr_fd() with
disk->open_mutex held causes lockdep warnings. We need a mechanism which
can flush outstanding I/O without disk->open_mutex held.

This post_release() operation may be called multiple times since multiple
threads may open the same path concurrently.

Also, this post_release() operation is called from only bdev_release()
path. This is because loop_configure() is not yet called (there is nothing
to clear) if something went wrong between an initialization lo_open() and
an error-unwinding lo_release() within the bdev_open() path.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
[ bvanassche: changed bdev->bd_disk into disk and removed those references
  to loop driver internals that are no longer correct ]
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/bdev.c                     | 3 +++
 include/linux/blkdev.h           | 8 ++++++++
 rust/kernel/block/mq/gen_disk.rs | 1 +
 3 files changed, 12 insertions(+)

diff --git a/block/bdev.c b/block/bdev.c
index fac74319e9fb..350f3c29d682 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -1189,6 +1189,9 @@ void bdev_release(struct file *bdev_file)
 		blkdev_put_whole(bdev);
 	mutex_unlock(&disk->open_mutex);
 
+	if (disk->fops->post_release)
+		disk->fops->post_release(disk);
+
 	module_put(disk->fops->owner);
 put_no_open:
 	blkdev_put_no_open(bdev);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index d003a9d2d1f6..5f51fe0d8bcc 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1580,6 +1580,14 @@ struct block_device_operations {
 			unsigned int flags);
 	int (*open)(struct gendisk *disk, blk_mode_t mode);
 	void (*release)(struct gendisk *disk);
+	/*
+	 * This operation is called after returned from release() and
+	 * disk->open_mutex was released. But this operation is not called
+	 * after an initialization open() has succeeded but something went
+	 * wrong and an error-unwinding release() was called.
+	 * This operation might sleep and has to be idempotent.
+	 */
+	void (*post_release)(struct gendisk *disk);
 	int (*ioctl)(struct block_device *bdev, blk_mode_t mode,
 			unsigned cmd, unsigned long arg);
 	int (*compat_ioctl)(struct block_device *bdev, blk_mode_t mode,
diff --git a/rust/kernel/block/mq/gen_disk.rs b/rust/kernel/block/mq/gen_disk.rs
index fc97dd873974..2ff77ef49781 100644
--- a/rust/kernel/block/mq/gen_disk.rs
+++ b/rust/kernel/block/mq/gen_disk.rs
@@ -129,6 +129,7 @@ pub fn build<T: Operations>(
             submit_bio: None,
             open: None,
             release: None,
+            post_release: None,
             ioctl: None,
             compat_ioctl: None,
             check_events: None,