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

[Paolo Bonzini]

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

Paolo