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Date: Sat, 2 Jul 2022 13:38:40 -0700 From: Andrew Morton <akpm@...ux-foundation.org> To: Sander Vanheule <sander@...nheule.net> Cc: x86@...nel.org, linux-kernel@...r.kernel.org, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, elver@...gle.com, gregkh@...uxfoundation.org, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, vschneid@...hat.com, Yury Norov <yury.norov@...il.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "H . Peter Anvin" <hpa@...or.com> Subject: Re: [PATCH v4 0/5] cpumask: Fix invalid uniprocessor assumptions On Sat, 2 Jul 2022 18:08:23 +0200 Sander Vanheule <sander@...nheule.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/
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