Re: [PATCH 1/2] Implement basic sleeping locks for gnumach.

[Agustina Arzille]

On 2017-01-29 21:34, Samuel Thibault wrote:
> Agustina Arzille, on Sun 29 Jan 2017 21:11:12 -0300, wrote:
> > On 2017-01-29 17:44, Samuel Thibault wrote:
> > >Agustina Arzille, on Sun 29 Jan 2017 16:46:45 -0300, wrote:
> > >
> > >There is just one thing I'm a bit afraid of: the code is assuming that
> > >the only reason why the locker thread is awakened is that it was given
> > >the mutex control. Is that guaranteed?
> >
> > Well, the locker thread checks the wait_result value: If it's
> > THREAD_AWAKENED,
> > it assumes that control of the mutex was transferred to it; otherwise, 
> > it
> > assumes it was interrupted. I *think* that's good enough.
>
> But what if some other thread, completely unrelated to the mutex, wakes
> the locker thread, for whatever reason unrelated to the mutex?

That case isn't covered. That said, such thread would have to do a 
clear_wait
on the locker thread with a result of THREAD_AWAKENED, and that seems a 
bit
malicious to me.


> The sleep queue is already protected inside assert_wait with a
> simple_lock.
>
> What you however want to serialize together is changing the mutex state
> and the wakeup.

Right.

> Yes, "any" here is important, I'd say, otherwise one could think that
> it's only one thread we have missed, while we actually have checked for
> all of them.

Good point. The next patch will be more clear :)