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

From: Mathieu Desnoyers
Date: Tue Nov 21 2017 - 17:04:09 EST

----- On Nov 21, 2017, at 12:21 PM, Andi Kleen andi@xxxxxxxxxxxxxx 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.



Mathieu Desnoyers
EfficiOS Inc.