linux-kernel - Re: [RFC PATCH for 4.15 v12 00/22] Restartable sequences and CPU op vector

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Date:   Tue, 21 Nov 2017 22:05:08 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.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>, Andrew Hunter <ahh@...gle.com>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        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>
Subject: Re: [RFC PATCH for 4.15 v12 00/22] Restartable sequences and CPU op
 vector

----- On Nov 21, 2017, at 12:21 PM, Andi Kleen andi@...stfloor.org wrote:

> On Tue, Nov 21, 2017 at 09:18:38AM -0500, Mathieu Desnoyers wrote:
>> Hi,
>> 
>> Following changes based on a thorough coding style and patch changelog
>> review from Thomas Gleixner and Peter Zijlstra, I'm respinning this
>> series for another RFC.
>> 
> My suggestion would be that you also split out the opv system call.
> That seems to be main contention point currently, and the restartable
> sequences should be useful without it.

I consider rseq to be incomplete and a pain to use in various scenarios
without cpu_opv. 

About the contention point you refer to:

Using vDSO as an example of how things should be done is just wrong: the
vDSO interaction with debugger instruction single-stepping is broken,
as I detailed in my previous email.

Thomas' proposal of handling single-stepping with a user-space locking
fallback, which is pretty much what I had in 2016, pushes a lot of
complexity to user-space, requires an extra branch in the fast-path,
as well as additional store-release/load-acquire semantics for consistency.
I don't plan going down that route.

Other than that, I have not received any concrete alternative proposal to
properly handle single-stepping.

The only opposition against cpu_opv is that there *should* be an hypothetical
simpler solution. The rseq idea is not new: it's been presented by Paul Turner
in 2012 at LPC. And so far, cpu_opv is the overall simplest and most
efficient way I encountered to handle single-stepping, and it gives extra
benefits, as described in my changelog.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com