linux-kernel - Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

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Date:	Mon, 2 Jun 2014 15:55:57 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <waiman.long@...com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	John David Anglin <dave.anglin@...l.net>,
	Parisc List <linux-parisc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Peter Anvin <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Jason Low <jason.low2@...com>
Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in
 cancelable mcs spinlocks

On Mon, Jun 2, 2014 at 3:08 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> rcu: Eliminate read-modify-write ACCESS_ONCE() calls
>
>         preempt_disable();
> -       ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
> +       lp = this_cpu_ptr(&sp->per_cpu_ref->c[idx]);
> +       ACCESS_ONCE(*lp) = *lp + 1;
>         smp_mb(); /* B */  /* Avoid leaking the critical section. */
> -       ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
> +       lp = this_cpu_ptr(&sp->per_cpu_ref->seq[idx]);
> +       ACCESS_ONCE(*lp) = *lp + 1;
>         preempt_enable();
>         return idx;

What Eric said. This should just use "this_cpu_inc()" instead.
Particularly with the smp_mb() and the preempt_enable(), there's no
way that could/should leak, and the ACCESS_ONCE() seems pointless and
ugly.

And the good news is, gcc _will_ generate good code for that.

              Linus
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