Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

[Nick Piggin]

On Fri, May 02, 2008 at 02:59:29PM +0200, Peter Zijlstra wrote:
> On Fri, 2008-05-02 at 05:42 -0700, Paul E. McKenney wrote:
> 
> > And here is one scenario that makes me doubt that my imagination is
> > faulty:
> > 
> > 1.	CPU 0 disables irqs.
> > 
> > 2.	CPU 1 disables irqs.
> > 
> > 3.	CPU 0 invokes smp_call_function().  But CPU 1 will never respond
> > 	because its irqs are disabled.
> > 
> > 4.	CPU 1 invokes smp_call_function().  But CPU 0 will never respond
> > 	because its irqs are disabled.
> > 
> > Looks like inherent deadlock to me, requiring that smp_call_function()
> > be invoked with irqs enabled.
> > 
> > So, what am I missing here?
> 
> The wish to do it anyway ;-)
> 
> I can imagine some situations where I'd like to try anyway and fall back
> to a slower path when failing.
> 
> With the initial design we would simply allocate data, stick it on the
> queue and call the ipi (when needed).
> 
> This is perfectly deadlock free when wait=0 and it just returns -ENOMEM
> on allocation failure.

Yeah, I'm just talking about the wait=0 case. (btw. I'd rather the core
API takes some data rather than allocates some itself, eg because you
might want to have it on the stack).

For the wait=1 case, something very clever such as processing pending
requests in a polling loop might be cool... however I'd rather not add
such complexity until someone needs it (you could stick a comment in
there outlining your algorithm). But I'd just rather not have peole rely
on it yet.

 
> It it doesn't return -ENOMEM I know its been queued and will be
> processed at some point, if it does fail, I can deal with it in another
> way.

At least with IPIs I think we can guarantee they will be processed on
the target after we queue them.

 
> I know I'd like to do that and I suspect Nick has a few use cases up his
> sleeve as well.

It would be handy. The "quickly kick something off on another CPU" is
pretty nice in mm/ when you have per-cpu queues or caches that might
want to be flushed.
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