linux-kernel - Re: [PATCH V2] x86/intel_rdt: Ensure usage of CPUs are locked while needed

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Date:   Tue, 11 Dec 2018 19:50:59 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, tony.luck@...el.com,
        gavin.hindman@...el.com, jithu.joseph@...el.com, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH V2] x86/intel_rdt: Ensure usage of CPUs are locked while
 needed

On Tue, Dec 11, 2018 at 10:33:59AM -0800, Reinette Chatre wrote:
> I am not sure that this is an issue when updating a schemata in the
> general case. In the case when just CAT schemata (without
> pseudo-locking) is updated then the cpu mask associated with the cache
> instance is indeed used to determine which CPUs should have their
> registers changed but only the current CPU is not checked for being
> online, for the other CPUs smp_call_function_many() is used that
> includes an online check.

Well, in your fix rdtgroup_schemata_write() disables hotplug for its
whole duration and doesn't look at what schemata update is being done,
right?

> I had the same question in V1's notes to the maintainer :)

Whoops, and I read that... Sorry. :-\

> My initial concern was the lack of IS_ERR checking. Understanding the
> flow better now it seems to me that this is indeed not a bug now. The
> reasoning is that an ERR_PTR is only returned when a negative id is
> provided in the parameters to rdt_find_domain(). There are currently
> only two places where a negative id could be provided to
> rdt_find_domain(), domain_add_cpu() and domain_remove_cpu(), and both
> locations test the return value using IS_ERR.

Right. I'll queue it for the normal merge window.

Thx.

-- 
Regards/Gruss,
    Boris.

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