Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

[Hugh Dickins <hughd@google.com>]

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On Mon, 18 Jul 2011, Linus Torvalds wrote:
> On Mon, Jul 18, 2011 at 12:47 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Huh?  We do __d_drop() in there, and do that before we start messing
> > with ->d_inode.
> 
> Hmm. Yes, looking at it, the ordering all seems correct. But then what
> did Hugh see at all?
> 
> The inode thing he got from d_inode is re-verified by
> __d_lookup_rcu(). So if inode is NULL, that means that the other CPU
> has done dentry_iput(), which means that __d_drop has already
> happened, which means that the dentry has been removed from the hash
> list *and* the count has been incremented.

__d_lookup_rcu() is being careful about *inode, yes.

But I'd forgotten it was even setting it: doesn't that setting get
overridden later by the more careless *inode = path->d_entry->d_inode
at the head of __follow_mount_rcu()'s loop?

Perhaps that line just needs to be moved to the tail of the loop?

Hugh