fs,block: get holder during claim

Now that we open block devices as files we need to deal with the
realities that closing is a deferred operation. An operation on the
block device such as e.g., freeze, thaw, or removal that runs
concurrently with umount, tries to acquire a stable reference on the
holder. The holder might already be gone though. Make that reliable by
grabbing a passive reference to the holder during bdev_open() and
releasing it during bdev_release().

Fixes: f3a608827d ("bdev: open block device as files") # mainline only
Reported-by: Christoph Hellwig <hch@infradead.org>
Link: https://lore.kernel.org/r/ZfEQQ9jZZVes0WCZ@infradead.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Reported-by: https://lore.kernel.org/r/CAHj4cs8tbDwKRwfS1=DmooP73ysM__xAb2PQc6XsAmWR+VuYmg@mail.gmail.com
Link: https://lore.kernel.org/r/20240315-freibad-annehmbar-ca68c375af91@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

block/bdev.c

@@ -583,6 +583,9 @@ static void bd_finish_claiming(struct block_device *bdev, void *holder,
 	mutex_unlock(&bdev->bd_holder_lock);
 	bd_clear_claiming(whole, holder);
 	mutex_unlock(&bdev_lock);
+
+	if (hops && hops->get_holder)
+		hops->get_holder(holder);
 }
 
 /**
@@ -605,6 +608,7 @@ EXPORT_SYMBOL(bd_abort_claiming);
 static void bd_end_claim(struct block_device *bdev, void *holder)
 {
 	struct block_device *whole = bdev_whole(bdev);
+	const struct blk_holder_ops *hops = bdev->bd_holder_ops;
 	bool unblock = false;
 
 	/*
@@ -627,6 +631,9 @@ static void bd_end_claim(struct block_device *bdev, void *holder)
 		whole->bd_holder = NULL;
 	mutex_unlock(&bdev_lock);
 
+	if (hops && hops->put_holder)
+		hops->put_holder(holder);
+
 	/*
 	 * If this was the last claim, remove holder link and unblock evpoll if
 	 * it was a write holder.

fs/super.c

@@ -1515,11 +1515,29 @@ static int fs_bdev_thaw(struct block_device *bdev)
 	return error;
 }
 
+static void fs_bdev_super_get(void *data)
+{
+	struct super_block *sb = data;
+
+	spin_lock(&sb_lock);
+	sb->s_count++;
+	spin_unlock(&sb_lock);
+}
+
+static void fs_bdev_super_put(void *data)
+{
+	struct super_block *sb = data;
+
+	put_super(sb);
+}
+
 const struct blk_holder_ops fs_holder_ops = {
 	.mark_dead		= fs_bdev_mark_dead,
 	.sync			= fs_bdev_sync,
 	.freeze			= fs_bdev_freeze,
 	.thaw			= fs_bdev_thaw,
+	.get_holder		= fs_bdev_super_get,
+	.put_holder		= fs_bdev_super_put,
 };
 EXPORT_SYMBOL_GPL(fs_holder_ops);
 

include/linux/blkdev.h

@@ -1505,6 +1505,16 @@ struct blk_holder_ops {
 	 * Thaw the file system mounted on the block device.
 	 */
 	int (*thaw)(struct block_device *bdev);
+
+	/*
+	 * If needed, get a reference to the holder.
+	 */
+	void (*get_holder)(void *holder);
+
+	/*
+	 * Release the holder.
+	 */
+	void (*put_holder)(void *holder);
 };
 
 /*