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

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Date:	Mon, 9 Sep 2013 01:30:26 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Waiman Long <Waiman.Long@...com>,
	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 Mon, Sep 09, 2013 at 01:03:00AM +0100, Al Viro wrote:

> Well...  unlazy_walk() is always followed by terminate_walk() very shortly,
> but there's a minor problem - terminate_walk() uses "are we in RCU
> mode?" for two things:
> 	a) do we need to do path_put() here?
> 	b) do we need to unlock?
> If you introduce the third case ("no need to do unlock and no need to
> do path_put()"), we'd better decide how to check for that case...
> 
> I suspect that minimal variant would be along the lines of
> 	* have unlazy_walk() slap NULL into ->path.mnt on error, clear
> LOOKUP_RCU and unlock
> 	* have terminate_walk() check ->path.mnt before doing path_put()
> in !RCU case
> 	* in do_last() replace bool got_write with struct vfsmount *got_write,
> storing the reference to vfsmount we'd fed to mnt_want_write().
> And use its value when we call mnt_put_write() in there...
> 
> I'll put together a commit like that on top of what I was going to push
> into public queues tonight; give me about half an hour, OK?

See the last commit in vfs.git#for-next (38373e1).
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