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

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Date:	Wed, 16 Jun 2010 19:48:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	paulus <paulus@...ba.org>,
	stephane eranian <eranian@...glemail.com>,
	Robert Richter <robert.richter@....com>,
	Will Deacon <will.deacon@....com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Yanmin <yanmin_zhang@...ux.intel.com>,
	Deng-Cheng Zhu <dengcheng.zhu@...il.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 3/8] perf: register pmu implementations

On Wed, 2010-06-16 at 19:03 +0200, Frederic Weisbecker wrote:

> > +static struct pmu perf_breakpoint = {
> > +	.event_init	= hw_breakpoint_event_init,
> 
> 
> 
> Should be bp_perf_event_init?

Ah, yes, like said, the compiler didn't get near yet..

> 
> > +	.enable		= arch_install_hw_breakpoint,
> > +	.disable	= arch_uninstall_hw_breakpoint,
> > +	.read		= hw_breakpoint_pmu_read,
> > +};
> <snip>
> > +static int perf_swevent_int(struct perf_event *event)
> > +{
> > +	if (event->attr.type != PERF_TYPE_SOFTWARE)
> > +		return -ENOENT
> 
> 
> perf_swevent_init() ?

copy/paste gone wild..

> > +void perf_pmu_unregister(struct pmu *pmu)
> > +{
> > +	spin_lock(&pmus_lock);
> > +	list_del_rcu(&pmu->entry);
> > +	spin_unlock(&pmus_lock);
> > +
> > +	synchronize_srcu(&pmus_srcu);
> > +}

> Who needs this?

Nobody yet.. 

> > +
> > +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.
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