Re: [Qemu-devel] [PATCH 1/4] iothread: replace init_done_cond with a semaphore

[Paolo Bonzini]

On 22/02/19 07:36, Peter Xu wrote:
> On Fri, Feb 22, 2019 at 07:25:16AM +0100, Marc-André Lureau wrote:
>> Hi
>>
>> On Fri, Feb 22, 2019 at 4:14 AM Peter Xu <address@hidden> wrote:
>>>
>>> Only sending an init-done message using lock+cond seems an overkill to
>>> me.  Replacing it with a simpler semaphore.
>>>
>>> Meanwhile, init the semaphore unconditionally, then we can destroy it
>>> unconditionally too in finalize which seems cleaner.
>>>
>>> Signed-off-by: Peter Xu <address@hidden>
>>
>> The lock is also protecting thread_id.
> 
> IMHO it's fine because thread_id is only changed at the beginning of
> iothread_run where the caller will definitely wait for the thread_id
> to be generated.  Here qemu_sem_post() should at least contain one
> write memory barrier there to make sure the waker will read the
> correct value after sem_wait() and then later on thread_id is never
> changed.

Yes, qemu_sem_post() is a "release" operation.  Anything that happens
before it is visible to the thread that does qemu_sem_wait().

In fact, thread_id is accessed outside the lock in
iothread_instance_finalize, so Peter's change is overall an improvement.

Paolo