linux-kernel - Re: [PATCH 9/9] x86/apic: Use non-atomic operations when possible

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Date:   Wed, 26 Jun 2019 00:03:01 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>
cc:     Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 9/9] x86/apic: Use non-atomic operations when possible

On Tue, 25 Jun 2019, Dave Hansen wrote:

> > diff --git a/arch/x86/kernel/apic/x2apic_cluster.c b/arch/x86/kernel/apic/x2apic_cluster.c
> > index 7685444a106b..609e499387a1 100644
> > --- a/arch/x86/kernel/apic/x2apic_cluster.c
> > +++ b/arch/x86/kernel/apic/x2apic_cluster.c
> > @@ -50,7 +50,7 @@ __x2apic_send_IPI_mask(const struct cpumask *mask, int vector, int apic_dest)
> >  	cpumask_copy(tmpmsk, mask);
> >  	/* If IPI should not be sent to self, clear current CPU */
> >  	if (apic_dest != APIC_DEST_ALLINC)
> > -		cpumask_clear_cpu(smp_processor_id(), tmpmsk);
> > +		__cpumask_clear_cpu(smp_processor_id(), tmpmsk);
> 
> tmpmsk is on-stack, but it's a pointer to a per-cpu variable:
> 
>         tmpmsk = this_cpu_cpumask_var_ptr(ipi_mask);
> 
> So this one doesn't appear as obviously correct as a mask which itself
> is on the stack.  The other three look obviously OK, though.

It's still correct. The mask is per cpu and protected because interrupts
are disabled. I noticed and wanted to amend the change log and forgot.

Thanks,

	tglx