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

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Message-ID: <20160407153948.GH24771@twins.programming.kicks-ass.net>
Date:	Thu, 7 Apr 2016 17:39:48 +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 05:24:32PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 07, 2016 at 07:35:26AM -0700, Andy Lutomirski wrote:
> > That way we could take an async signal, handle it, and resume, even in
> > the middle of a commit, without aborting.  Of course, if the signal
> > hander tried to access the same rseq-protected resource, it would bump
> > the event counter and cause an abort.
> 
> Ah, so what happens if the signal happens before the commit but after
> the load of the seqcount?
> 
> Then, even if the signal motifies the count, we'll not observe.

Ah, and the same is true for preemptions. Which is why all this was
preemption driven, and not migration driven.