[PATCH v2 10/10] fs: Fix missed inode writeback when racing with __writeback_single_inode

[Jan Kara <jack@suse.cz>]

When mmb_fsync_noflush() or simple_fsync_noflush() race with another
writeback of the same inode, they can see inode dirty bits are already
clear and skip inode writeback although the racing
__writeback_single_inode() didn't yet get to writing anything. This can
result in fsync(2) returning without properly persisting the inode.

We already have I_SYNC bit for this synchronization and
writeback_single_inode() properly uses it so just fix
mmb_fsync_noflush() and simple_fsync_noflush() to take it into account
as well.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/buffer.c | 5 +++--
 fs/libfs.c  | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index f83fb3cdc6ac..01e17a287a42 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -662,9 +662,10 @@ int mmb_fsync_noflush(struct file *file, struct mapping_metadata_bhs *mmb,
 	if (err)
 		return err;
 
-	if (!(inode_state_read_once(inode) & I_DIRTY_ALL))
+	if (!(inode_state_read_once(inode) & (I_DIRTY_ALL | I_SYNC)))
 		goto sync_buffers;
-	if (datasync && !(inode_state_read_once(inode) & I_DIRTY_DATASYNC))
+	if (datasync &&
+	    !(inode_state_read_once(inode) & (I_DIRTY_DATASYNC | I_SYNC)))
 		goto sync_buffers;
 
 	ret = sync_inode_metadata(inode, 1);
diff --git a/fs/libfs.c b/fs/libfs.c
index 1bbea5e7bae3..3a98dacd81b2 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1559,9 +1559,10 @@ int simple_fsync_noflush(struct file *file, loff_t start, loff_t end,
 	if (err)
 		return err;
 
-	if (!(inode_state_read_once(inode) & I_DIRTY_ALL))
+	if (!(inode_state_read_once(inode) & (I_DIRTY_ALL | I_SYNC)))
 		goto out;
-	if (datasync && !(inode_state_read_once(inode) & I_DIRTY_DATASYNC))
+	if (datasync &&
+	    !(inode_state_read_once(inode) & (I_DIRTY_DATASYNC | I_SYNC)))
 		goto out;
 
 	ret = sync_inode_metadata(inode, 1);
-- 
2.51.0