[PATCH 7/7] block: Don't block on s_umount from __invalidate_super()

[Kent Overstreet]

__invalidate_super() is used to flush any filesystem mounted on a
device, generally on some sort of media change event.

However, when unmounting a filesystem and closing the underlying block
devices, we can deadlock if the block driver then calls
__invalidate_device() (e.g. because the block device goes away when it
is no longer in use).

This happens with bcachefs on top of loopback, and can be triggered by
fstests generic/042:

  put_super
    -> blkdev_put
    -> lo_release
    -> disk_force_media_change
    -> __invalidate_device
    -> get_super

This isn't inherently specific to bcachefs - it hasn't shown up with
other filesystems before because most other filesystems use the sget()
mechanism for opening/closing block devices (and enforcing exclusion),
however sget() has its own downsides and weird/sketchy behaviour w.r.t.
block device open lifetime - if that ever gets fixed more code will run
into this issue.

The __invalidate_device() call here is really a best effort "I just
yanked the device for a mounted filesystem, please try not to lose my
data" - if it's ever actually needed the user has already done something
crazy, and we probably shouldn't make things worse by deadlocking.
Switching to a trylock seems in keeping with what the code is trying to
do.

If we ever get revoke() at the block layer, perhaps we would look at
rearchitecting to use that instead.

Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 block/bdev.c       |  2 +-
 fs/super.c         | 40 +++++++++++++++++++++++++++++++---------
 include/linux/fs.h |  1 +
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/block/bdev.c b/block/bdev.c
index 1795c7d4b9..743e969b7b 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -922,7 +922,7 @@ EXPORT_SYMBOL(lookup_bdev);
 
 int __invalidate_device(struct block_device *bdev, bool kill_dirty)
 {
-	struct super_block *sb = get_super(bdev);
+	struct super_block *sb = try_get_super(bdev);
 	int res = 0;
 
 	if (sb) {
diff --git a/fs/super.c b/fs/super.c
index 04bc62ab7d..a2decce02f 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -791,14 +791,7 @@ void iterate_supers_type(struct file_system_type *type,
 
 EXPORT_SYMBOL(iterate_supers_type);
 
-/**
- * get_super - get the superblock of a device
- * @bdev: device to get the superblock for
- *
- * Scans the superblock list and finds the superblock of the file system
- * mounted on the device given. %NULL is returned if no match is found.
- */
-struct super_block *get_super(struct block_device *bdev)
+static struct super_block *__get_super(struct block_device *bdev, bool try)
 {
 	struct super_block *sb;
 
@@ -813,7 +806,12 @@ struct super_block *get_super(struct block_device *bdev)
 		if (sb->s_bdev == bdev) {
 			sb->s_count++;
 			spin_unlock(&sb_lock);
-			down_read(&sb->s_umount);
+
+			if (!try)
+				down_read(&sb->s_umount);
+			else if (!down_read_trylock(&sb->s_umount))
+				return NULL;
+
 			/* still alive? */
 			if (sb->s_root && (sb->s_flags & SB_BORN))
 				return sb;
@@ -828,6 +826,30 @@ struct super_block *get_super(struct block_device *bdev)
 	return NULL;
 }
 
+/**
+ * get_super - get the superblock of a device
+ * @bdev: device to get the superblock for
+ *
+ * Scans the superblock list and finds the superblock of the file system
+ * mounted on the device given. %NULL is returned if no match is found.
+ */
+struct super_block *get_super(struct block_device *bdev)
+{
+	return __get_super(bdev, false);
+}
+
+/**
+ * try_get_super - get the superblock of a device, using trylock on sb->s_umount
+ * @bdev: device to get the superblock for
+ *
+ * Scans the superblock list and finds the superblock of the file system
+ * mounted on the device given. %NULL is returned if no match is found.
+ */
+struct super_block *try_get_super(struct block_device *bdev)
+{
+	return __get_super(bdev, true);
+}
+
 /**
  * get_active_super - get an active reference to the superblock of a device
  * @bdev: device to get the superblock for
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c85916e9f7..1a6f951942 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2878,6 +2878,7 @@ extern struct file_system_type *get_filesystem(struct file_system_type *fs);
 extern void put_filesystem(struct file_system_type *fs);
 extern struct file_system_type *get_fs_type(const char *name);
 extern struct super_block *get_super(struct block_device *);
+extern struct super_block *try_get_super(struct block_device *);
 extern struct super_block *get_active_super(struct block_device *bdev);
 extern void drop_super(struct super_block *sb);
 extern void drop_super_exclusive(struct super_block *sb);
-- 
2.40.1