Re: rseq + membarrier programming model

[Florian Weimer <fweimer@redhat.com>]

* Mathieu Desnoyers:

>> Could it fall back to
>> MEMBARRIER_CMD_GLOBAL instead?
>
> No. CMD_GLOBAL does not issue the required rseq fence used by the
> algorithm discussed. Also, CMD_GLOBAL has quite a few other shortcomings:
> it takes a while to execute, and is incompatible with nohz_full kernels.

What about using sched_setcpu to move the current thread to the same CPU
(and move it back afterwards)?  Surely that implies the required sort of
rseq barrier that MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ with
MEMBARRIER_CMD_FLAG_CPU performs?

That is possible even without membarrier, so I wonder why registration
of intent is needed for MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ.

> In order to make sure the programming model is the same for expedited
> private/global plain/sync-core/rseq membarrier commands, we require that
> each process perform a registration beforehand.

Hmm.  At least it's not possible to unregister again.

But I think it would be really useful to have some of these barriers
available without registration, possibly in a more expensive form.

Thanks,
Florian