fs: stop using get_super in fs_mark_dead

fs_mark_dead currently uses get_super to find the superblock for the
block device that is going away.  This means it is limited to the
main device stored in sb->s_dev, leading to a lot of code duplication
for file systems that can use multiple block devices.

Now that the holder for all block devices used by file systems is set
to the super_block, we can instead look at that holder and then check
if the file system is born and active, so do that instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Message-Id: <20230802154131.2221419-8-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/super.c b/fs/super.c
index 3c7e463..9cf7fc6 100644 (file)
--- a/fs/super.c
+++ b/fs/super.c
@@ -1209,17 +1209,39 @@ int get_tree_keyed(struct fs_context *fc,
 EXPORT_SYMBOL(get_tree_keyed);
 
 #ifdef CONFIG_BLOCK
+/*
+ * Lock a super block that the callers holds a reference to.
+ *
+ * The caller needs to ensure that the super_block isn't being freed while
+ * calling this function, e.g. by holding a lock over the call to this function
+ * and the place that clears the pointer to the superblock used by this function
+ * before freeing the superblock.
+ */
+static bool lock_active_super(struct super_block *sb)
+{
+       down_read(&sb->s_umount);
+       if (!sb->s_root ||
+           (sb->s_flags & (SB_ACTIVE | SB_BORN)) != (SB_ACTIVE | SB_BORN)) {
+               up_read(&sb->s_umount);
+               return false;
+       }
+       return true;
+}
+
 static void fs_mark_dead(struct block_device *bdev)
 {
-       struct super_block *sb;
+       struct super_block *sb = bdev->bd_holder;
 
-       sb = get_super(bdev);
-       if (!sb)
+       /* bd_holder_lock ensures that the sb isn't freed */
+       lockdep_assert_held(&bdev->bd_holder_lock);
+
+       if (!lock_active_super(sb))
                return;
 
        if (sb->s_op->shutdown)
                sb->s_op->shutdown(sb);
-       drop_super(sb);
+
+       up_read(&sb->s_umount);
 }
 
 static const struct blk_holder_ops fs_holder_ops = {