Re: [PATCH][RFC]: mutex: adaptive spin

Steven Rostedt
2009-01-06T18:20:44Z

On Tue, 6 Jan 2009, Linus Torvalds wrote:

>
> Ok, last comment, I promise.
>
> On Tue, 6 Jan 2009, Peter Zijlstra wrote:
> > @@ -175,11 +199,19 @@ __mutex_lock_common(struct mutex *lock,
> > debug_mutex_free_waiter(&waiter);
> > return -EINTR;
> > }
> > - __set_task_state(task, state);
> >
> > - /* didnt get the lock, go to sleep: */
> > + owner = lock->owner;
> > + get_task_struct(owner);
> > spin_unlock_mutex(&lock->wait_lock, flags);
> > - schedule();
> > +
> > + if (adaptive_wait(&waiter, owner, state)) {
> > + put_task_struct(owner);
> > + __set_task_state(task, state);
> > + /* didnt get the lock, go to sleep: */
> > + schedule();
> > + } else
> > + put_task_struct(owner);
> > +
> > spin_lock_mutex(&lock->wait_lock, flags);
>
> So I really dislike the whole get_task_struct/put_task_struct thing. It
> seems very annoying. And as far as I can tell, it's there _only_ to
> protect "task->rq" and nothing else (ie to make sure that the task
> doesn't exit and get freed and the pointer now points to la-la-land).

Yeah, that was not one of the things that we liked either. We tried
other ways to get around the get_task_struct but, ended up with
the get_task_struct in the end anyway.

>
> Wouldn't it be much nicer to just cache the rq pointer (take it while
> still holding the spinlock), and then pass it in to adaptive_wait()?
>
> Then, adaptive_wait() can just do
>
> if (lock->owner != owner)
> return 0;
>
> if (rq->task != owner)
> return 1;
>
> Sure - the owner may have rescheduled to another CPU, but if it did that,
> then we really might as well sleep. So we really don't need to dereference
> that (possibly stale) owner task_struct at all - because we don't care.
> All we care about is whether the owner is still busy on that other CPU
> that it was on.
>
> Hmm? So it looks to me that we don't really need that annoying "try to
> protect the task pointer" crud. We can do the sufficient (and limited)
> sanity checking without the task even existing, as long as we originally
> load the ->rq pointer at a point where it was stable (ie inside the
> spinlock, when we know that the task must be still alive since it owns the
> lock).

Caching the rq is an interesting idea. But since the rq struct is local to
sched.c, what would be a good API to do this?

in mutex.c:

void *rq;

[...]

rq = get_task_rq(owner);
spin_unlock(&lock->wait_lock);

[...]

if (!task_running_on_rq(rq, owner))

in sched.c:

void *get_task_rq(struct task_struct *p)
{
return task_rq(p);
}

int task_running_on_rq(void *r, struct task_sturct *p)
{
struct rq *rq = r;

return rq->curr == p;
}

??

-- Steve