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Date: Fri, 7 Jul 2017 08:50:40 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Shivappa Vikas <vikas.shivappa@intel.com>
cc: Vikas Shivappa <vikas.shivappa@linux.intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, hpa@zytor.com, peterz@infradead.org,
        ravi.v.shankar@intel.com, tony.luck@intel.com, fenghua.yu@intel.com,
        andi.kleen@intel.com
Subject: Re: [PATCH 21/21] x86/intel_rdt/mbm: Handle counter overflow
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On Thu, 6 Jul 2017, Shivappa Vikas wrote:
> On Sun, 2 Jul 2017, Thomas Gleixner wrote:
> > On Mon, 26 Jun 2017, Vikas Shivappa wrote:
> > > +static void mbm_update(struct rdt_domain *d, int rmid)
> > > +{
> > > +	struct rmid_read rr;
> > > +
> > > +	rr.first = false;
> > > +	rr.d = d;
> > > +
> > > +	if (is_mbm_total_enabled()) {
> > > +		rr.evtid = QOS_L3_MBM_TOTAL_EVENT_ID;
> > > +		__mon_event_count(rmid, &rr);
> > 
> > This is broken as it is not protected against a concurrent read from user
> > space which comes in via a smp function call.
> 
> The read from user also has the rdtgroup_mutex.

Which is again, completely non obvious and undocumented in the code.

Aside of that, are you really serious about serializing the world and
everything on a single global mutex?

Thanks,

	tglx