linux-kernel - Re: NFS/d_splice

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Message-ID: <20160603032853.GK14480@ZenIV.linux.org.uk>
Date:	Fri, 3 Jun 2016 04:28:53 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Trond Myklebust <trondmy@...marydata.com>
Cc:	Oleg Drokin <green@...uxhacker.ru>,
	"J. Bruce Fields" <bfields@...hat.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"<linux-kernel@...r.kernel.org> Mailing List" 
	<linux-kernel@...r.kernel.org>,
	"<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>
Subject: Re: NFS/d_splice_alias breakage

On Fri, Jun 03, 2016 at 12:44:51AM +0000, Trond Myklebust wrote:

> That would have to be a really tight race, since the code in _nfs4_open_and_get_state() currently reads:
> 
>                 d_drop(dentry);
>                 alias = d_exact_alias(dentry, state->inode);
>                 if (!alias)
>                         alias = d_splice_alias(igrab(state->inode), dentry);
> 
> IOW: something would have to be acting between the d_drop() and d_splice_alias() above...

How?  dentry is
	* negative (it would better be, or we are _really_ fucked)
	* unhashed

How does whoever's rehashing it stumble across that thing?