Re: [Qemu-devel] [PATCH 2/3] coroutine: Clean up qemu_coroutine_enter()

[Kevin Wolf <kwolf@redhat.com>]

Am 10.02.2015 um 11:55 hat Paolo Bonzini geschrieben:
> 
> 
> On 10/02/2015 11:41, Kevin Wolf wrote:
> > +    ret = qemu_coroutine_switch(self, co, COROUTINE_ENTER);
> > +
> > +    qemu_co_queue_run_restart(co);
> > +
> > +    switch (ret) {
> > +    case COROUTINE_YIELD:
> > +        return;
> > +    case COROUTINE_TERMINATE:
> > +        trace_qemu_coroutine_terminate(co);
> > +        coroutine_delete(co);
> > +        return;
> > +    default:
> 
> Say you have:
> 
>   co1                                     co2
> ------------------------------------------------------------------------
> 1 qemu_co_mutex_lock(&m);
> 2 qemu_coroutine_yield();
> 3                                         qemu_co_mutex_lock(&m);
> 4 qemu_co_mutex_unlock(&m);
> 5 qemu_coroutine_yield();
> 
> Then you have:
> 
> 1 mutex->locked = true;
> 
> 2 coroutine_swap(co1, leader, COROUTINE_YIELD);
> 
> 3 while (mutex->locked) {
>      qemu_co_queue_wait(&mutex->queue);
>            '--> QTAILQ_INSERT_TAIL(&queue->entries, self, co_queue_next);
>                 qemu_coroutine_yield();
>                 '--> coroutine_swap(co2, leader, COROUTINE_YIELD);
>   }
> 
> 4 mutex->locked = false;
>   qemu_co_queue_next(&mutex->queue);
>    '--> qemu_co_queue_do_restart(queue, true);
>         '--> QTAILQ_REMOVE(&queue->entries, next, co_queue_next);
>              QTAILQ_INSERT_TAIL(&self->co_queue_wakeup, next, co_queue_next);
> 
> 5 coroutine_swap(co1, leader, COROUTINE_YIELD);
> 
> And co2 is never reentered until co1 terminates.  Right?

No, co2 will be reentered during the yield in line 5. However, it's not
the yielding coroutine that reenters it but the parent, which is resumed
at exactly the line of code that you quoted above.

This is actually how it always worked, even with the bug. The bug caused
it to access the queue of a random other coroutine, but that queue must
have always been empty because it was already processed when that other
coroutine yielded/terminated.

Kevin