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 08:57:53 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
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>, Chris Lameter <cl@...ux.com>,
	Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Dave Watson <davejwatson@...com>,
	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 Apr 8, 2016 4:04 AM, "Peter Zijlstra" <peterz@...radead.org> wrote:
>
> On Thu, Apr 07, 2016 at 03:05:26PM -0700, Andy Lutomirski wrote:
>
> > It doesn't, which is what I like about my variant.  If the thread
> > accesses the protected data structure, though, it should bump the
> > sequence count, which will cause the first thread to about when it
> > gets scheduled in.
>
> Nope it won't, because that first thread is right at the commit
> instruction, nothing will stop it from executing that store and clobbing
> what we just wrote.
>

I don't think so.  I write an event number.  You commit because you
didn't notice.  I haven't loaded yet from the value you wrote when you
committed, so nothing goes wrong.

--Andy