Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

Michael S. Tsirkin
2009-06-16T14:55:02Z

On Tue, Jun 16, 2009 at 10:48:27AM -0400, Gregory Haskins wrote:
> >>>> +static void _eventfd_notify(struct eventfd_ctx *ctx)
> >>>> +{
> >>>> + struct eventfd_notifier *en;
> >>>> + int idx;
> >>>> +
> >>>> + idx = srcu_read_lock(&ctx->srcu);
> >>>> +
> >>>> + /*
> >>>> + * The goal here is to allow the notification to be preemptible
> >>>> + * as often as possible. We cannot achieve this with the basic
> >>>> + * wqh mechanism because it requires the wqh->lock. Therefore
> >>>> + * we have an internal srcu list mechanism of which the wqh is
> >>>> + * a client.
> >>>> + *
> >>>> + * Not all paths will invoke this function in process context.
> >>>> + * Callers should check for suitable state before assuming they
> >>>> + * can sleep (such as with preemptible()). Paul McKenney assures
> >>>> + * me that srcu_read_lock is compatible with in-atomic, as long as
> >>>> + * the code within the critical section is also compatible.
> >>>> + */
> >>>> + list_for_each_entry_rcu(en, &ctx->nh, list)
> >>>> + en->ops->signal(en);
> >>>> +
> >>>> + srcu_read_unlock(&ctx->srcu, idx);
> >>>> +}
> >>>> +
> >>>> /*
> >>>> * Adds "n" to the eventfd counter "count". Returns "n" in case of
> >>>> * success, or a value lower then "n" in case of coutner overflow.
> >>>>
> >>>>
> >>> This is ugly, isn't it? With CONFIG_PREEMPT=no preemptible() is always false.
> >>>
> >>> Further, to do useful things it might not be enough that you can sleep:
> >>> with iofd you also want to access current task with e.g. copy from user.
> >>>
> >>> Here's an idea: let's pass a flag to ->signal, along the lines of
> >>> signal_is_task, that tells us that it is safe to use current, and add
> >>> eventfd_signal_task() which is the same as eventfd_signal but lets everyone
> >>> know that it's safe to both sleep and use current->mm.
> >>>
> >>> Makes sense?
> >>>
> >>>
> >> It does make sense, yes. What I am not clear on is how would eventfd
> >> detect this state such as to populate such flags, and why cant the
> >> ->signal() CB do the same?
> >>
> >> Thanks Michael,
> >> -Greg
> >>
> >>
> >
> > eventfd can't detect this state. But the callers know in what context they are.
> > So the *caller* of eventfd_signal_task makes sure of this: if you are in a task,
> > you can call eventfd_signal_task() if not, you must call eventfd_signal.
> >
> >
> >
> Hmm, this is an interesting idea, but I think it would be problematic in
> real-world applications for the long-term. For instance, the -rt tree
> and irq-threads .config option in the process of merging into mainline
> changes context types for established code. Therefore, what might be
> "hardirq/softirq" logic today may execute in a kthread tomorrow.

That's OK, it's always safe to call eventfd_signal: eventfd_signal_task is just
an optimization. I think everyone not in the context of a system call or vmexit
can just call eventfd_signal_task.

> I
> think its dangerous to try to solve the problem with caller provided
> info: the caller may be ignorant of its true state.

I assume this wasn't clear enough: the idea is that you only
calls eventfd_signal_task if you know you are on a systemcall path.
If you are ignorant of the state, call eventfd_signal.

> IMO, the ideal
> solution needs to be something we can detect at run-time.
>
> Thanks Michael,
> -Greg
>

--
MST