Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> * Linus Torvalds (torvalds@linux-foundation.org) wrote:
> > >  - SA_RUNNING: a way to signal only running threads - as a way for user-space 
> > >    based concurrency control mechanisms to deschedule running threads (or, like
> > >    in your case, to implement barrier / garbage collection schemes).
> > 
> > Hmm. This sounds less fundamentally broken, but at the same time also 
> > _way_ more invasive in the signal handling layer. It's already one of our 
> > more "exciting" layers out there.
> > 
> 
> Hrm, thinking about it a bit further, the only way I see we could provide a
> usable SA_RUNNING flag would be to add hooks to the scheduler. These hooks would
> somehow have to call user-space code (!) when scheduling in/out a thread. Yes,
> this sounds utterly broken (since these hooks would have to be preemptable).
> 
> The idea is this: if we look, for instance, at the kernel preemptable RCU
> implementations, they consist of two parts: one is iteration on all CPUs to
> consider all active CPUs, and the other is a modification of the scheduler to
> note all preempted tasks that were in a preemptable RCU C.S..
> 
> Just for the memory barrier we consider for sys_membarrier(), I had to ensure
> that the scheduler issues memory barriers to order accesses to user-space memory
> and mm_cpumask modifications. In reality, what we are doing is to ensure that
> the operation required on the running thread is done by the scheduler too when
> scheduling in/out the task.
> 
> As soon as we have signal handlers which perform more than a simple memory
> barrier (e.g. something that has side-effects outside of the processor), I doubt
> it would ever make sense to only run the handler on running threads unless we
> have hooks in the scheduler too.

Unless this question is answered, Ingo's SA_RUNNING signal proposal, as
appealing as it may look at a first glance, falls into the "fundamentally
broken" category. I don't see any neat way to make the scheduler call into
user-space hooks to deal with inherent synchronization required between
iteration on active threads and scheduler activity. But who knows, maybe it's
just a lack of imagination from my part.

Thanks,

Mathieu

> 
> Thanks,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency Consultant
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
Operating System Efficiency Consultant
EfficiOS Inc.
http://www.efficios.com