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

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Message-ID: <20130830205404.GF13318@ZenIV.linux.org.uk>
Date:	Fri, 30 Aug 2013 21:54:04 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 01:43:11PM -0700, Linus Torvalds wrote:
> On Fri, Aug 30, 2013 at 1:15 PM, Waiman Long <waiman.long@...com> wrote:
> >
> > The prepend_path() isn't all due to getcwd. The correct profile should be
> 
> Ugh. I really think that prepend_path() should just be rewritten to
> run entirely under RCU.
> 
> Then we can remove *all* the stupid locking, and replace it with doing
> a read-lock on the rename sequence count, and repeating if requited.
> 
> That shouldn't even be hard to do, it just requires mindless massaging
> and being careful.

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...
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