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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 16, 2009 at 10:54:28AM -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.  I
> > think its dangerous to try to solve the problem with caller provided
> > info:  the caller may be ignorant of its true state.
> 
> Also, we need to consider that a process context can still be in-atomic
> if the user did something like disabled interrupts, preemption, used a
> spinlock, etc, before calling the eventfd_signal_task() function. 
> Perhaps we can put a stake in the ground that says you must not call
> this from atomic context,

That's the ticket.

> but I still prefer just being able to detect
> this from our state.
> 
> -Greg
> 
> 

-- 
MST