Re: [Qemu-devel] [PATCH 16/16] cpus-common: lock-free fast path for cpu_exec_start/end

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 24/09/2016 22:43, Richard Henderson wrote:
>>>> I don't see that the cpu_list_lock protects the
>>>> last two lines in any way.
>>>
>>> It does:
>>>
>>>         qemu_mutex_lock(&qemu_cpu_list_lock);
>>
>> What I meant is that I don't see that the mutex avoids the need for
>> atomic_set.
>
> Oh, I see.
>
> cpu->running is only read under the mutex, but can be written _by the
> owner thread only_ outside the mutex.  So writes outside the mutex must
> be atomic, but writes under the mutex don't because:
>
> - no other thread ever writes to cpu->running
>
> - no other thread can be reading cpu->running

Should we add some comments to cpu.h's definitions to make the rules clear?

--
Alex Bennée