linux-kernel - Re: [PATCH v5 0/5] cpumask: fix invalid uniprocessor assumptions

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Date:   Sun, 31 Jul 2022 15:02:55 +0200
From:   Sander Vanheule <sander@...nheule.net>
To:     Yury Norov <yury.norov@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Gow <davidgow@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Maíra Canal <mairacanal@...eup.net>,
        Marco Elver <elver@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v5 0/5] cpumask: fix invalid uniprocessor assumptions

On Sat, 2022-07-30 at 11:15 -0700, Yury Norov wrote:
> On Fri, Jul 29, 2022 at 09:01:17AM +0200, Sander Vanheule 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 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/
>  
> Hi Sander,
> 
> I tried to apply it on top of bitmap-for next, and there are many conflicts
> with already pulled patches. There's nothing really scary, just functions
> changed their prototypes and locations. Can you try your series on top of
> bitmap-for-next from git@...hub.com:/norov/linux.git (or just -next)? 
> 
> I'm asking you to do it instead of doing myself because I don't want to
> screwup your code accidentally and because many cpumask functions in -next
> are moved to the header, and it would be probably possible to avoid building 
> cpumask.o in UP case.
> 
> Briefly looking into the -next code, cpumask.c hosts  only cpumask_next_wrap()
> that is not overwritten by UP code, and in UP case it can be simplified.

Sure. I've rebased my patches and added a UP-version for cpumask_next_wrap(), so
cpumask.o doesn't have to be built anymore in that case.

How would you like to proceed with these patches? It's fine by me if you take
them through your tree to avoid more merge conflicts with your changes, but then
Andrew woud have to drop the series from mm-nonmm-stable.

Best,
Sander