linux-kernel - Re: [PATCH 1/3] x86/intel_rdt: Fix issue when mkdir uses a freed CLOSid

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Date:   Wed, 5 Apr 2017 17:20:24 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>
cc:     vikas.shivappa@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
        peterz@...radead.org, ravi.v.shankar@...el.com,
        tony.luck@...el.com, fenghua.yu@...el.com, h.peter.anvin@...el.com
Subject: Re: [PATCH 1/3] x86/intel_rdt: Fix issue when mkdir uses a freed
 CLOSid

On Mon, 3 Apr 2017, Vikas Shivappa wrote:

> Subject: x86/intel_rdt: Fix issue when mkdir uses a freed CLOSid

This subject line is useless again. It want's to be descriptive.

"Fix issue" Which issue?

> Each resctrl directory has one CLOSid allocated which is mapped to a
> control register/QOS_MSR. During an rmdir this CLOSid is freed and can
> be reused later when a new directory is created.  Currently we do not
> reset the QOS_MSR to a default when the CLOSid is freed. So when the
> next mkdir uses a freed CLOSid, the new directory is associated with a
> stale QOS_MSR.

That's not an issue. That's maybe something people are not expecting.

> Fix this issue by writing a default value to the QOS_MSR when the
> associated CLOSid is freed. The default value is all 1s for CBM which
> means no control is enforced when a mkdir reuses this CLOSid.

That's just wrong.

The proper behaviour for a new control group is, that at the time when it
is created it copies the CBM values of the default group and not claiming
access to ALL of the cache by default.

Thanks,

	tglx