Re: [RFC][PATCH 3/8] perf: register pmu implementations

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Jun 16, 2010 at 07:48:00PM +0200, Peter Zijlstra wrote:
> > > +
> > > +struct pmu *perf_init_event(struct perf_event *event)
> > > +{
> > > +	struct pmu *pmu;
> > > +	int idx;
> > > +
> > > +	idx = srcu_read_lock(&pmus_srcu);
> > > +	list_for_each_entry_rcu(pmu, &pmus, entry) {
> > > +		int ret = pmu->event_init(event);
> > > +		if (!ret)
> > > +			break;
> > > +		if (ret != -ENOENT) {
> > > +			pmu = ERR_PTR(ret);
> > > +			break;
> > >  		}
> > > -		pmu = &perf_ops_generic;
> > > -		break;
> > >  	}
> > > +	srcu_read_unlock(&pmus_srcu, idx);
> > 
> > 
> > 
> > This could use a simple mutex instead of a spinlock + srcu_sync on
> > writer and srcu on reader.
> 
> Right, that spinlock needs to be a mutex for sure, a later patch adds an
> allocation under it.
> 
> But even with a mutex we need srcu_sync in there to sync against the
> readers.
> 
> > That doesn't matter much that said. What I don't understand is
> > why we need to synchronize the writers. Walking the list with
> > list_*_rcu() looks justified once we support boot events, but
> > until then...
> 
> Well, the typical unregister user would be a module, if you unregister
> and then dealloc the struct pmu by unloading the module a reader might
> still see a reference to it if you don't srcu_sync it.


Ok, I see what you mean.