Re: [Qemu-ppc] [RFC v3 0/56] per-CPU locks

[Emilio G. Cota]

On Fri, Oct 19, 2018 at 08:59:24 +0200, Paolo Bonzini wrote:
> On 19/10/2018 03:05, Emilio G. Cota wrote:
> > I'm calling this series a v3 because it supersedes the two series
> > I previously sent about using atomics for interrupt_request:
> >   https://lists.gnu.org/archive/html/qemu-devel/2018-09/msg02013.html
> > The approach in that series cannot work reliably; using (locked) atomics
> > to set interrupt_request but not using (locked) atomics to read it
> > can lead to missed updates.
> 
> The idea here was that changes to protected fields are all followed by
> kick.  That may not have been the case, granted, but I wonder if the
> plan is unworkable.

I suspect that the cpu->interrupt_request+kick mechanism is not the issue,
otherwise master should not work--we do atomic_read(cpu->interrupt_request)
and only if that read != 0 we take the BQL.

My guess is that the problem is with other reads of cpu->interrupt_request,
e.g. those in cpu_has_work. Currently those reads happen with the
BQL held, and updates to cpu->interrupt_request take the BQL. If we drop
the BQL from the setters to instead use locked atomics (like in the
aforementioned series), those BQL-protected readers might miss updates.

Given that we need a per-CPU lock anyway to remove the BQL from the
CPU loop, extending this lock to protect cpu->interrupt_request is
a simple solution that keeps the current logic and allows for
greater scalability.

Thanks,

                Emilio