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

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Message-ID: <CALCETrXAtYDZBbpwZceFyhLOnqFmTDqTxhGfbrrVrY+34cxSFg@mail.gmail.com>
Date:	Wed, 24 Jun 2015 17:07:28 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paul Turner <pjt@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Hunter <ahh@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Josh Triplett <josh@...htriplett.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Lameter <cl@...ux.com>
Subject: Re: [RFC PATCH 0/3] restartable sequences: fast user-space percpu
 critical sections

On Wed, Jun 24, 2015 at 3:26 PM, Paul Turner <pjt@...gle.com> wrote:
> This is a fairly small series demonstrating a feature we've found to be quite
> powerful in practice, "restartable sequences".
>

On an extremely short glance, I'm starting to think that the right
approach, at least for x86, is to implement per-cpu gsbase.  Then you
could do cmpxchg with a gs prefix to atomically take a percpu lock and
atomically release a percpu lock and check whether someone else stole
the lock from you.  (Note: cmpxchg, unlike lock cmpxchg, is very
fast.)

This is totally useless for other architectures, but I think it would
be reasonable clean on x86.  Thoughts?

I can elaborate if necessary.

--Andy
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