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Date:   Mon, 18 May 2020 18:35:13 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Karol Herbst <karolherbst@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        nouveau@...ts.freedesktop.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH] x86: mmiotrace: Use cpumask_available for cpumask_var_t
 variables

On Mon, 18 May 2020 11:52:47 -0700
Nick Desaulniers <ndesaulniers@...gle.com> wrote:

> On Mon, May 18, 2020 at 2:31 AM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > On Wed, Apr 08, 2020 at 01:53:23PM -0700, Nathan Chancellor wrote:  
> > > When building with Clang + -Wtautological-compare and
> > > CONFIG_CPUMASK_OFFSTACK unset:
> > >
> > > arch/x86/mm/mmio-mod.c:375:6: warning: comparison of array 'downed_cpus'
> > > equal to a null pointer is always false [-Wtautological-pointer-compare]
> > >         if (downed_cpus == NULL &&
> > >             ^~~~~~~~~~~    ~~~~
> > > arch/x86/mm/mmio-mod.c:405:6: warning: comparison of array 'downed_cpus'
> > > equal to a null pointer is always false [-Wtautological-pointer-compare]
> > >         if (downed_cpus == NULL || cpumask_weight(downed_cpus) == 0)
> > >             ^~~~~~~~~~~    ~~~~
> > > 2 warnings generated.
> > >
> > > Commit f7e30f01a9e2 ("cpumask: Add helper cpumask_available()") added
> > > cpumask_available to fix warnings of this nature. Use that here so that
> > > clang does not warn regardless of CONFIG_CPUMASK_OFFSTACK's value.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/982
> > > Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> > > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>  
> 
> Thanks for the patch, sorry I'm falling behind on code review!
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Linus sent me a issue about this failure privately as well, and had two
solutions for it (one being identical to this one, and I shared that with
him, and another one he thought would be better, but had some issues).

Linus,

Are you OK with this patch?

-- Steve


> 
> > > ---
> > >  arch/x86/mm/mmio-mod.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/mm/mmio-mod.c b/arch/x86/mm/mmio-mod.c
> > > index 109325d77b3e..43fd19b3f118 100644
> > > --- a/arch/x86/mm/mmio-mod.c
> > > +++ b/arch/x86/mm/mmio-mod.c
> > > @@ -372,7 +372,7 @@ static void enter_uniprocessor(void)
> > >       int cpu;
> > >       int err;
> > >
> > > -     if (downed_cpus == NULL &&
> > > +     if (!cpumask_available(downed_cpus) &&
> > >           !alloc_cpumask_var(&downed_cpus, GFP_KERNEL)) {
> > >               pr_notice("Failed to allocate mask\n");
> > >               goto out;
> > > @@ -402,7 +402,7 @@ static void leave_uniprocessor(void)
> > >       int cpu;
> > >       int err;
> > >
> > > -     if (downed_cpus == NULL || cpumask_weight(downed_cpus) == 0)
> > > +     if (!cpumask_available(downed_cpus) || cpumask_weight(downed_cpus) == 0)
> > >               return;
> > >       pr_notice("Re-enabling CPUs...\n");
> > >       for_each_cpu(cpu, downed_cpus) {
> > >
> > > base-commit: ae46d2aa6a7fbe8ca0946f24b061b6ccdc6c3f25
> > > --
> > > 2.26.0
> > >  
> >
> > Gentle ping for acceptance, I am not sure who should take this.  
> 
> Looks like Steven or Ingo are the listed maintainers for MMIOTRACE?
> 

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