block: split bdev_yield_claim() out of bdev_fput()

bdev_fput() yields the holder claim and then closes the file, which is a
deferred operation.  Split the yield half into bdev_yield_claim() so a caller
can give up the holder while the file - and therefore the block device - is
still open, act on the device, and only then bdev_fput().

A filesystem that made a device unfreezable for a membership change with
bdev_deny_freeze() undoes the deny on release with

	bdev_yield_claim(bdev_file);
	bdev_allow_freeze(file_bdev(bdev_file));
	bdev_fput(bdev_file);

Re-allowing only after the holder is yielded avoids stranding the filesystem
on a racing freeze, and doing it while the file is still open avoids touching
the block device after bdev_fput().  bdev_fput() yields again, which is a
no-op once the claim has already been given up.

Link: https://patch.msgid.link/20260616-work-super-freeze_deny_upstream-v2-2-b3567c7f994b@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewd-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 34 insertions, 16 deletions

diff --git a/block/bdev.c b/block/bdev.c
index 9b73487a91ca..28b0d40c362f 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -1196,6 +1196,39 @@ put_no_open:
 }
 
 /**
+ * bdev_yield_claim - give up the holder claim on an open block device
+ * @bdev_file: open block device
+ *
+ * Yield the holder and any write access for @bdev_file without closing it, so
+ * the caller can still act on the device - e.g. bdev_allow_freeze() it - before
+ * the final bdev_fput().  bdev_fput() yields too, so calling it afterwards is
+ * safe.
+ */
+void bdev_yield_claim(struct file *bdev_file)
+{
+	struct block_device *bdev;
+	struct gendisk *disk;
+
+	if (!bdev_file->private_data)
+		return;
+
+	bdev = file_bdev(bdev_file);
+	disk = bdev->bd_disk;
+
+	mutex_lock(&disk->open_mutex);
+	bdev_yield_write_access(bdev_file);
+	bd_yield_claim(bdev_file);
+	/*
+	 * Tell release we already gave up our hold on the
+	 * device and if write restrictions are available that
+	 * we already gave up write access to the device.
+	 */
+	bdev_file->private_data = BDEV_I(bdev_file->f_mapping->host);
+	mutex_unlock(&disk->open_mutex);
+}
+EXPORT_SYMBOL_GPL(bdev_yield_claim);
+
+/**
  * bdev_fput - yield claim to the block device and put the file
  * @bdev_file: open block device
  *
@@ -1208,22 +1241,7 @@ void bdev_fput(struct file *bdev_file)
 	if (WARN_ON_ONCE(bdev_file->f_op != &def_blk_fops))
 		return;
 
-	if (bdev_file->private_data) {
-		struct block_device *bdev = file_bdev(bdev_file);
-		struct gendisk *disk = bdev->bd_disk;
-
-		mutex_lock(&disk->open_mutex);
-		bdev_yield_write_access(bdev_file);
-		bd_yield_claim(bdev_file);
-		/*
-		 * Tell release we already gave up our hold on the
-		 * device and if write restrictions are available that
-		 * we already gave up write access to the device.
-		 */
-		bdev_file->private_data = BDEV_I(bdev_file->f_mapping->host);
-		mutex_unlock(&disk->open_mutex);
-	}
-
+	bdev_yield_claim(bdev_file);
 	fput(bdev_file);
 }
 EXPORT_SYMBOL(bdev_fput);