On Tue, Oct 06, 2015 at 06:04:50PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 21, 2015 at 07:46:11PM +0200, Oleg Nesterov wrote:
> > On 09/18, Peter Zijlstra wrote:
> > >
> > > the text is correct, right?
> >
> > Yes, it looks good to me and helpful.
> >
> > But damn. I forgot why exactly try_to_wake_up() needs rmb() after
> > ->on_cpu check... It looks reasonable in any case, but I do not
> > see any strong reason immediately.
>
> I read it like the smp_rmb() we have for
> acquire__after_spin_is_unlocked. Except, as you note below, we need to
> need an smp_read_barrier_depends for control barriers as well....
> Yes, but I'm not sure we should go write:
>
> while (READ_ONCE_CTRL(p->on_cpu))
> cpu_relax();
>
> Or:
>
> while (p->on_cpu)
> cpu_relax();
>
> smp_read_barrier_depends();
>
> It seems to me that doing the smp_mb() (for Alpha) inside the loop might
> be sub-optimal.
And also referring to:
lkml.kernel.org/r/20150812133109.GA8266@xxxxxxxxxx
Do we want something like this?
#define smp_spin_acquire(cond) do { \
while (cond) \
cpu_relax(); \
smp_read_barrier_depends(); /* ctrl */ \
smp_rmb(); /* ctrl + rmb := acquire */ \
} while (0)
And use it like:
smp_spin_acquire(raw_spin_is_locked(&task->pi_lock));
That might work for your task_work_run() and the scheduler case,
although it might be somewhat awkward for sem_wait_array().
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