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 01:35:17 +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 Sun, Sep 08, 2013 at 05:25:40PM -0700, Linus Torvalds wrote:
> On Sun, Sep 8, 2013 at 5:03 PM, Al Viro <viro@...iv.linux.org.uk> 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...
> 
> Actually, I decided to take advantage of those two cases instead, and
> I have a patch that I think does the right thing. Basically, I start
> off unlazy_walk() with just doing that lockref_get_not_dead() on the
> parent dentry, and if that fails I just return an error in RCU mode
> (so terminate_walk() does what it always used to do, and we haven't
> done anything else to any refcounts).
> 
> Now, if the lockref_get_not_dead() succeeded, that means that we have
> a reference on the nd->path.dentry, and we can now just do
> "mntget(nd->path.mnt);". Ta-Daa! We now have everything done for the
> non-RCU case for terminate_walk().
> 
> So after that point, we clear LOOKUP_RCU, and make the rule be that
> any return (error or success) has to do unlock_rcu_walk(). And then
> all the other refcounts are easy, we can just "dput(dentry);" after
> that.
> 
> I haven't tested it yet, I was going to reboot into it just now. But
> I'm attaching the patch here. Maybe I missed some detail, but it all
> seems simpler.
> 
> Note that this patch requires the "lockref_get_not_dead()" cleanup at
> the top of my current -git.

That should also work, replacing the current tip of #for-next.  Do you
prefer to merge those two diffs of yours into a single commit?
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