linux-kernel - Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections

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Date:	Fri, 8 Apr 2016 08:41:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Turner <commonly@...il.com>,
	Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
	Dave Watson <davejwatson@...com>,
	Josh Triplett <josh@...htriplett.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Hunter <ahh@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu
 critical sections

On Thu, Apr 07, 2016 at 09:43:33AM -0700, Andy Lutomirski wrote:
> enter the critical section:
> 1:
> movq %[cpu], %%r12
> movq {address of counter for our cpu}, %%r13
> movq {some fresh value}, (%%r13)
> cmpq %[cpu], %%r12
> jne 1b

This is inherently racy; your forgot the detail of 'some fresh value',
but since you want to avoid collisions you really want an increment.

But load-store archs cannot do that. Or rather, they need to do:

	load	Rn, $event
	add	Rn, Rn, 1
	store	$event, Rn

But if they're preempted in the middle, two threads will collide and
generate the _same_ increment. Comparing CPU numbers will not fix that.