linux-kernel - Re: [RFC PATCH v7 1/7] Restartable sequences system call

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Message-ID: <alpine.DEB.2.20.1608031330000.14875@east.gentwo.org>
Date:	Wed, 3 Aug 2016 13:31:30 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Ben Maurer <bmaurer@...com>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call

On Wed, 3 Aug 2016, Andy Lutomirski wrote:

> > Well, a CMPXCHG without LOCK prefix isn't all that expensive on x86.
> >
> > It is however on PPC and possibly other architectures, so in name of
> > simplicity supporting only the one variant makes sense.
> >
>
> I wouldn't want to depend on CMPXCHG.  But imagine we had primitives
> that were narrower than the full abort-on-preemption primitive.
> Specifically, suppose we had abort if (actual cpu != expected_cpu ||
> *aptr != aval).  We could do things like:
>

The latency issues that are addressed by restartable sequences require
minimim instruction overhead. Lockless CMPXCHG is very important in that
area and I would not simply remove it from consideration.