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

[Ingo Molnar <mingo@elte.hu>]

* Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> * 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. [...]

How is it different from your syscall? I.e. which lines of code make the 
difference? We could certainly apply the (trivial) barrier change to 
context_switch().

	Ingo