From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH 1/3] perf: add context field to perf_event
Date: Thu, 21 Jul 2011 16:46:48 +0100
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Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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	Ingo Molnar <mingo@elte.hu>,
	"acme@ghostprotocols.net" <acme@ghostprotocols.net>,
	Jason Wessel <jason.wessel@windriver.com>
To: Avi Kivity <avi@redhat.com>
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On Thu, Jul 21, 2011 at 04:36:43PM +0100, Avi Kivity wrote:
> On 07/21/2011 06:32 PM, Will Deacon wrote:
> > >
> > >  Using TIF_bits sounds like a much better solution for this, wakeups are
> > >  really rather expensive and its best to avoid extra if at all possible.
> >
> > The problem with using a TIF bit to tell a task that it needs to perform
> > some preempt_notifier registrations is that you end up with something that
> > looks a lot like preempt notifiers! You also don't escape the concurrent
> > read/write to thelist of pending registrations.
> >
> > One thing I tried was simply using an RCU protected hlist for the preempt
> > notifiers so that we don't have to worry about atomicity when reading the
> > notifiers in finish_task_switch. It's a bit odd, since we know we only ever
> > have a single reader, but I've included it below anyway.
> >
> > If anybody has any better ideas, I'm all ears.
> 
> > +void preempt_notifier_register_task(struct preempt_notifier *notifier,
> > +				    struct task_struct *tsk)
> > +{
> > +	mutex_lock(&tsk->preempt_notifiers_mutex);
> > +	hlist_add_head_rcu(&notifier->link,&tsk->preempt_notifiers);
> > +	mutex_unlock(&tsk->preempt_notifiers_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(preempt_notifier_register_task);
> > +
> > +void preempt_notifier_unregister_task(struct preempt_notifier *notifier,
> > +				      struct task_struct *tsk)
> > +{
> > +	mutex_lock(&tsk->preempt_notifiers_mutex);
> > +	hlist_del_rcu(&notifier->link);
> > +	mutex_unlock(&tsk->preempt_notifiers_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(preempt_notifier_unregister_task);
> > +
> >   /**
> >    * preempt_notifier_register - tell me when current is being preempted&  rescheduled
> >    * @notifier: notifier struct to register
> >    */
> >   void preempt_notifier_register(struct preempt_notifier *notifier)
> >   {
> > -	hlist_add_head(&notifier->link,&current->preempt_notifiers);
> > +	preempt_notifier_register_task(notifier, current);
> >   }
> >   EXPORT_SYMBOL_GPL(preempt_notifier_register);
> 
> This is (and must be) called from a preempt disabled context, no mutexes 
> around here.

Bah, yes, that is essential if you're dealing with current. Maybe use a
spinlock instead?

Will