Re: [qemu-s390x] [RFC v3 0/56] per-CPU locks

[Paolo Bonzini]

On 19/10/2018 16:50, Emilio G. Cota wrote:
> 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.

cpu_has_work is only needed to handle the processor's halted state (or
is it?).  If it is, OR+kick should work.

> 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.

Sure, I was just curious what the problem was.  KVM uses OR+kick with no
problems.

Paolo