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Subject: Re: [PATCH V2] x86/intel_rdt: Ensure usage of CPUs are locked while
 needed
To:     Borislav Petkov <bp@alien8.de>
Cc:     tglx@linutronix.de, fenghua.yu@intel.com, tony.luck@intel.com,
        gavin.hindman@intel.com, jithu.joseph@intel.com, mingo@redhat.com,
        hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
References: <b7b17432a80f95a1fa21a1698ba643014f58ad31.1544476425.git.reinette.chatre@intel.com>
 <20181211123404.GC27375@zn.tnic>
 <ece2581e-d5e5-c26c-53e5-0cef8b37e59f@intel.com>
 <20181211185059.GP27375@zn.tnic>
From:   Reinette Chatre <reinette.chatre@intel.com>
Message-ID: <e4ef2e81-defd-c5ca-4892-a0361a4528ba@intel.com>
Date:   Tue, 11 Dec 2018 11:02:08 -0800
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Hi Boris,

On 12/11/2018 10:50 AM, Borislav Petkov wrote:
> 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?

Correct.

I just wanted to emphasize that it is not the schemata writing that
needs to be protected, but instead the pseudo-locking code that runs
after schemata programming that needs to run on a particular CPU. The
new patch subject could be interpreted to mean the former ... but that
is starting to sound like nitpicking by me.

>> I had the same question in V1's notes to the maintainer :)
> 
> Whoops, and I read that... Sorry. :-\

Not a problem at all :)

>> 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.

Thank you very much.

Reinette