linux-kernel - Re: [4.9-rc1+] intel_uncore builtin + CONFIG_DEBUG_TEST_DRIVER

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Date:   Thu, 20 Oct 2016 11:42:59 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     CAI Qian <caiqian@...hat.com>, Rob Herring <robh@...nel.org>,
        Kan Liang <kan.liang@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [4.9-rc1+] intel_uncore builtin +
 CONFIG_DEBUG_TEST_DRIVER_REMOVE kernel panic

On Thu, Oct 20, 2016 at 11:04:16AM +0200, Peter Zijlstra wrote:
> On Thu, Oct 20, 2016 at 10:58:03AM +0200, Jiri Olsa wrote:
> 
> > @@ -8869,11 +8869,15 @@ void perf_pmu_unregister(struct pmu *pmu)
> >  	free_percpu(pmu->pmu_disable_count);
> >  	if (pmu->type >= PERF_TYPE_MAX)
> >  		idr_remove(&pmu_idr, pmu->type);
> > -	if (pmu->nr_addr_filters)
> > -		device_remove_file(pmu->dev, &dev_attr_nr_addr_filters);
> > -	device_del(pmu->dev);
> > -	put_device(pmu->dev);
> > +	mutex_lock(&pmus_lock);
> > +	if (pmu_bus_running) {
> > +		if (pmu->nr_addr_filters)
> > +			device_remove_file(pmu->dev, &dev_attr_nr_addr_filters);
> > +		device_del(pmu->dev);
> > +		put_device(pmu->dev);
> > +	}
> >  	free_pmu_context(pmu);
> > +	mutex_unlock(&pmus_lock);
> >  }
> >  EXPORT_SYMBOL_GPL(perf_pmu_unregister);
> 
> I think that is still racy..
> 
> 
> unregister:		sysfs_init:
> 
> mutex_lock(&pmus_lock);
> list_del_rcu(&pmu->entry);
> mutex_unlock(&pmus_lock);
> 
> synchronize_*rcu();
> 
> 			mutex_lock(&pmus_lock);
> 			list_for_each_entry(pmu, &pmus, entry) {
> 				/* add device muck */

ah, I thought this part would add the device back.. but it's
already out of the pmu list.. right :-\

thanks,
jirka

> 				/* will _NOT_ see our PMU */
> 			}
> 			pmus_bus_running = 1;
> 			mutex_unlock(&pmus_lock);
> 
> mutex_lock(&pmus_lock);
> if (pmu_bus_running) {
> 	device_del() /* OOPS */
> 
> 
> What you want is to read pmu_bus_running in the same pmus_lock section
> as we do the list_del, and then use that local copy later.