linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Message-ID: <CA+55aFwwKEaTBX9Uoexpote7dOHBq+TiJ=RifzTpVL_jh24A9w@mail.gmail.com>
Date:	Fri, 30 Aug 2013 14:03:59 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Waiman Long <waiman.long@...com>, Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Fri, Aug 30, 2013 at 1:54 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Not really.  Sure, you'll retry it if you race with d_move(); that's not
> the real problem - access past the end of the object containing ->d_name.name
> would screw you and that's what ->d_lock is preventing there.  Delayed freeing
> of what ->d_name is pointing into is fine, but it's not the only way to get
> hurt there...

Umm? We follow d->d_name.name without d_lock under RCU all the time -
that's what the pathname lookup is all about, after all.

Yes, yes, you haev to be careful and cannot just blindly trust the
length: you also have to check for NUL character as you are copying it
and stop if you hit it. But that's trivial.

Why would d_prepend be any different?

            Linus
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