[PATCH] vfs: Write inodes reliably before calling ->write_super() and ->sync_fs()

[Jan Kara]

So far, __fsync_super() and do_sync() called sync_inodes(sb, 0), then called
->write_super(), ->sync_fs() and after that called sync_inodes(sb, 1). This
ordering makes it kind of hard for filesystems as sync_inodes(sb, 0) need not
submit all the IO (for example it skips inodes with I_SYNC set) so e.g. forcing
transaction to disk in ->sync_fs() is not really enough. Yes, it means sys_sync
has not been completely reliable on some filesystems (ext3, ext4, reiserfs,
ocfs2 and others are hit by this) when racing e.g. with background writeback. A
similar problem hits also other filesystems (e.g. ext2) because of
write_super() being called before the sync_inodes(sb, 1). Change the ordering
so that inodes are first reliably written out and write_super() and sync_fs()
are called only after that.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/super.c |    2 +-
 fs/sync.c  |    9 +++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 786fe7d..4c12918 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -267,6 +267,7 @@ void __fsync_super(struct super_block *sb)
 {
 	sync_inodes_sb(sb, 0);
 	vfs_dq_sync(sb);
+	sync_inodes_sb(sb, 1);
 	lock_super(sb);
 	if (sb->s_dirt && sb->s_op->write_super)
 		sb->s_op->write_super(sb);
@@ -274,7 +275,6 @@ void __fsync_super(struct super_block *sb)
 	if (sb->s_op->sync_fs)
 		sb->s_op->sync_fs(sb, 1);
 	sync_blockdev(sb->s_bdev);
-	sync_inodes_sb(sb, 1);
 }
 
 /*
diff --git a/fs/sync.c b/fs/sync.c
index 7abc65f..4de4c89 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -26,10 +26,15 @@ static void do_sync(unsigned long wait)
 	wakeup_pdflush(0);
 	sync_inodes(0);		/* All mappings, inodes and their blockdevs */
 	vfs_dq_sync(NULL);
-	sync_supers();		/* Write the superblocks */
 	sync_filesystems(0);	/* Start syncing the filesystems */
+	/*
+	 * We have to reliably submit IO for all the inodes before writing
+	 * super blocks and calling sync_fs(). Otherwise superblock could miss
+	 * some updates or journal could still have uncommitted data.
+	 */
+	sync_inodes(wait);	
+	sync_supers();		/* Write the superblocks */
 	sync_filesystems(wait);	/* Waitingly sync the filesystems */
-	sync_inodes(wait);	/* Mappings, inodes and blockdevs, again. */
 	if (!wait)
 		printk("Emergency Sync complete\n");
 	if (unlikely(laptop_mode))
-- 
1.6.0.2


--pWyiEgJYm5f9v55/--
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