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-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org wrote:

On Tue, Nov 21, 2017 at 2:05 PM, Mathieu Desnoyers
[off-list ref] wrote:

quoted

----- On Nov 21, 2017, at 12:21 PM, Andi Kleen andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org wrote:

quoted

On Tue, Nov 21, 2017 at 09:18:38AM -0500, Mathieu Desnoyers wrote:

quoted

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