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

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Date:	Sun, 8 Sep 2013 17:25:40 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
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 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.

        Linus

Download attachment "patch.diff" of type "application/octet-stream" (4618 bytes)