linux-ext4 - [PATCH 07/12] fs: stop using get_super in fs_mark

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Date:   Wed,  2 Aug 2023 17:41:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-block@...r.kernel.org
Subject: [PATCH 07/12] 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@....de>
---
 fs/super.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 09b65ee1a8b737..0cda4af0a7e16c 100644
--- 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 = {
-- 
2.39.2