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

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Date:   Wed, 22 Nov 2017 16:43:43 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        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 22, 2017, at 10:28 AM, Andy Lutomirski luto@...capital.net wrote:

> On Tue, Nov 21, 2017 at 2:05 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>> ----- 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.
>>
> 
> If anyone ever reports that as a problem, I'll gladly fix it in the
> kernel.  That's doable without an ABI change.  If rseq-like things
> started breaking single-stepping, we can't just fix it in the kernel.

Very true. And rseq does break both line-level and instruction-level
single-stepping.

> 
> Also, there is one and only one vclock_gettime.  Debuggers can easily
> special-case it.  For all I know, they already do.

As my tests demonstrate, they don't. clock_gettime() vDSO currently
breaks instruction-level single-stepping (istep) with gdb. I'll
forward you the writeup I did on that a few days ago.

Thanks,

Mathieu

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