On Fri 24-06-16 05:36:05, Al Viro wrote:
> Could somebody explain when would the second part of that test _not_ be true?
>
> if (!ret && !hlist_empty(&inode->i_dentry))
> ret = ext4_sync_parent(inode);
>
> inode is that of an opened file; how could it possibly _not_ have a dentry
> alias? Is that code actually supposed to check if the sucker is not
> unlinked?
Yes, that's what the test was supposed to do I believe.
> If so, it's not what we are actually checking - pinned dentry remains
> positive (and unhashed) after unlink(2). What's more, the loop in
> ext4_sync_parent() is vulnerable to races with rmdir(2) - if you get
> unlink and rmdir of ancestors between
>
> next = igrab(d_inode(dentry->d_parent));
> and
> inode = next;
> ret = sync_mapping_buffers(inode->i_mapping);
> if (ret)
> break;
> ret = sync_inode_metadata(inode, 1);
> if (ret)
> break;
> you are risking interesting things done in the middle of rmdir and/or
> unlink; that might be actually safe, but in that case it's worth a comment
> explaining that.
You are right and it should be harmless. I'm now testing the patch below.
Thanks for your comments.
>From 3dbea3e4e58d9ad81f38e8f89f9f569daef5a800 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@xxxxxxx>
Date: Wed, 6 Jul 2016 13:55:58 +0200
Subject: [PATCH] ext4: Cleanup ext4_sync_parent()
A condition !hlist_empty(&inode->i_dentry) is always true for open file.
Just remove it. Also ext4_sync_parent() could use some explanation why
races with rmdir() are not an issue - add a comment explaining that.
Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/ext4/fsync.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index 8850254136ae..9b9335796fbc 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -61,6 +61,13 @@ static int ext4_sync_parent(struct inode *inode)
break;
iput(inode);
inode = next;
+ /*
+ * The directory inode may have gone through rmdir by now. But
+ * the inode itself and its blocks are still allocated (we hold
+ * a reference to the inode so it didn't go through
+ * ext4_evict_inode()) and so we are safe to flush metadata
+ * blocks and the inode.
+ */
ret = sync_mapping_buffers(inode->i_mapping);
if (ret)
break;
@@ -107,7 +114,7 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
if (!journal) {
ret = generic_file_fsync(file, start, end, datasync);
- if (!ret && !hlist_empty(&inode->i_dentry))
+ if (!ret)
ret = ext4_sync_parent(inode);
goto out;
}
--
2.6.6
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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