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Re: [Qemu-devel] [PATCH 1/4] iothread: replace init_done_cond with a sem
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 1/4] iothread: replace init_done_cond with a semaphore |
Date: |
Fri, 22 Feb 2019 10:27:01 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 |
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
[Qemu-devel] [PATCH 3/4] iothread: create main loop unconditionally, Peter Xu, 2019/02/21
[Qemu-devel] [PATCH 2/4] iothread: create the gcontext onconditionally, Peter Xu, 2019/02/21
[Qemu-devel] [PATCH 4/4] iothread: push gcontext earlier in the thread_fn, Peter Xu, 2019/02/21