linux-kernel - Re: [PATCH] x86, mm: Send tlb flush IPIs to online cpus only

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Date:	Thu, 19 Jul 2012 15:32:29 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	msb@...omium.org, mingo@...nel.org, linux-kernel@...r.kernel.org,
	shaohua.li@...el.com, yinghai@...nel.org, tglx@...utronix.de,
	hpa@...or.com, x86@...nel.org, tj@...nel.org,
	akpm@...ux-foundation.org, sfr@...b.auug.org.au, cl@...two.org,
	olofj@...omium.org, paulmck@...ux.vnet.ibm.com,
	Alex Shi <alex.shi@...el.com>,
	Conny Seidel <conny.seidel@....com>
Subject: Re: [PATCH] x86, mm: Send tlb flush IPIs to online cpus only

On Thu, Jul 19, 2012 at 06:27:33PM +0530, Srivatsa S. Bhat wrote:

[ … ]
 
> So we are sending an IPI to a cpu which is now offline. Once a cpu is offline,
> it will no longer respond to IPIs. This explains the softlockup.
> 
> A cpu in the mm_cpumask could go offline before we send the invalidate
> IPI causing us to wait forever. Avoid this by sending the IPI to only the
> online cpus.
> 
> [Since flush_tlb_others_ipi() is always called with preempt disabled, it is
> not possible for a CPU to go offline once we enter this function, because
> CPU offline goes through the stop_machine() stuff (which cannot proceed until
> all preempt disabled sections are exited). So we don't have to worry about
> any race between CPU offline and the target cpumask calculation in
> flush_tlb_others_ipi().]
> 
> Addresses http://crosbug.com/31737
> 
> Reported-and-debugged-by: Mandeep Singh Baines <msb@...omium.org>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> Acked-by: Mandeep Singh Baines <msb@...omium.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Stephen Rothwell <sfr@...b.auug.org.au>
> Cc: Christoph Lameter <cl@...two.org>
> Cc: Olof Johansson <olofj@...omium.org>
> ---
> 
>  arch/x86/mm/tlb.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 5e57e11..9d387a9 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -186,7 +186,11 @@ static void flush_tlb_others_ipi(const struct cpumask *cpumask,
>  
>  	f->flush_mm = mm;
>  	f->flush_va = va;
> -	if (cpumask_andnot(to_cpumask(f->flush_cpumask), cpumask, cpumask_of(smp_processor_id()))) {
> +
> +	cpumask_and(to_cpumask(f->flush_cpumask), cpumask, cpu_online_mask);
> +	cpumask_clear_cpu(smp_processor_id(), to_cpumask(f->flush_cpumask));
> +
> +	if (!cpumask_empty(to_cpumask(f->flush_cpumask))) {

FWIW, there's code in tip/x86/mm which reworks all that and
flush_tlb_others_ipi along with the 32 TLB flush vectors are being
removed in favor of a smp_call_function_many thing. And it should be
hotplug-safe since it must be called with preemption disabled anyway.

-- 
Regards/Gruss,
Boris.
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