Re: [Qemu-devel] [PATCH v7 1/5] cpu: Provide vcpu throttling interface

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/09/2015 13:01, Juan Quintela wrote:
> Paolo Bonzini <pbonzini@redhat.com> wrote:
>> On 09/09/2015 12:41, Juan Quintela wrote:
>>>>> +    qemu_mutex_unlock_iothread();
>>>>> +    atomic_set(&cpu->throttle_thread_scheduled, 0);
>>>>> +    g_usleep(sleeptime_ns / 1000); /* Convert ns to us for usleep call */
>>>>> +    qemu_mutex_lock_iothread();
>>>
>>> Why is this thread safe?
>>>
>>> qemu_mutex_lock_iothread() is protecting (at least) cpu_work_first on
>>> each cpu.  How can we be sure that _nothing_ will change that while we
>>> are waiting?
>>
>> You only have to be sure that the queued work list remains consistent;
>> not that nothing changes.
> 
> 
> But nothing else is protected by the iothread?

Not at this point.  Notice how qemu_kvm_wait_io_event calls
qemu_cond_wait just before qemu_wait_io_event_common (which in turn is
what calls flush_queued_work).

So you can be quite sure that qemu_wait_io_event_common runs at a point
where there's nothing hidden that relies on the iothread mutex.

Paolo