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Date:   Sat, 2 Jul 2022 13:38:40 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Sander Vanheule <sander@svanheule.net>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        elver@google.com, gregkh@linuxfoundation.org,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, vschneid@redhat.com,
        Yury Norov <yury.norov@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v4 0/5] cpumask: Fix invalid uniprocessor assumptions
Message-Id: <20220702133840.943817a7694406a135bb48a9@linux-foundation.org>
In-Reply-To: <cover.1656777646.git.sander@svanheule.net>
References: <cover.1656777646.git.sander@svanheule.net>
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On Sat,  2 Jul 2022 18:08:23 +0200 Sander Vanheule <sander@svanheule.net> wrote:

> On uniprocessor builds, it is currently assumed that any cpumask will
> contain the single CPU: cpu0. This assumption is used to provide
> optimised implementations.
> 
> The current assumption also appears to be wrong, by ignoring the fact
> that users can provide empty cpumask-s. This can result in bugs as
> explained in [1].

It's a little unkind to send people off to some link to explain the
very core issue which this patchset addresses!  So I enhanced this
paragraph:

: The current assumption also appears to be wrong, by ignoring the fact that
: users can provide empty cpumasks.  This can result in bugs as explained in
: [1] - for_each_cpu() will run one iteration of the loop even when passed
: an empty cpumask.

> This series introduces some basic tests, and updates the optimisations
> for uniprocessor builds.
> 
> The x86 patch was written after the kernel test robot [2] ran into a
> failed build. I have tried to list the files potentially affected by the
> changes to cpumask.h, in an attempt to find any other cases that fail on
> !SMP. I've gone through some of the files manually, and ran a few cross
> builds, but nothing else popped up. I (build) checked about half of the
> potientally affected files, but I do not have the resources to do them
> all. I hope we can fix other issues if/when they pop up later.
> 
> [1] https://lore.kernel.org/all/20220530082552.46113-1-sander@svanheule.net/
> [2] https://lore.kernel.org/all/202206060858.wA0FOzRy-lkp@intel.com/