Re: fs/namei.c: Misuse of sequence counts?

From: Al Viro
Date: Sat Oct 11 2014 - 19:46:41 EST

On Sat, Oct 11, 2014 at 05:58:08PM -0500, Eric Biggers wrote:

> In follow_dotdot_rcu(), said commit moved loads of the inode to just before
> read_seqcount_begin(), in several instances. I don't think this is correct,
> because (as I understand it) read_seqcount_begin() is opening a seq-read
> critical section on the new dentry. So the inode load should come *after* it,
> as in the original, to ensure the inode pointer is correctly matched with the
> sequence count.

Nope. What we do is
* pick parent inode and seqcount (in whatever order)
* THEN check that child is still unchanged.
The second part guarantees that parent dentry had been the parent of
child all along, since the moment we'd first fetched _child's_ seqcount.
And since a pinned positive dentry can't have its ->d_inode changed,
we know that the value of parent's inode we'd fetched remained valid
at least until we'd checked the child's seqcount and found it unchanged.
Which means that we had it valid at some point after we'd fetched parent's

The crucial part is that dentry cannot change its ->d_inode for as long as
there are references to it.
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