linux-ext4 - [RFC] weirdness in ext4_sync

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Message-ID: <20160624043604.GU14480@ZenIV.linux.org.uk>
Date:	Fri, 24 Jun 2016 05:36:05 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [RFC] weirdness in ext4_sync_file()

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?  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.
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