linux-kernel - Re: [PATCH] powerpc: Make doorbell check preemption safe

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Date:	Wed, 20 May 2015 11:00:08 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com>
Cc:	mpe@...erman.id.au, benh@...nel.crashing.org, paulus@...ba.org,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Make doorbell check preemption safe

On Wed, 2015-05-20 at 00:30 +0530, Shreyas B. Prabhu wrote:
> Doorbell can be used to cause ipi on cpus which are sibling threads on
> the same core. So icp_native_cause_ipi checks if the destination cpu
> is a sibling thread of the current cpu and uses doorbell in such cases.
> 
> But while running with CONFIG_PREEMPT=y, since this section is
> preemtible, we can run into issues if after we check if the destination
> cpu is a sibling cpu, the task gets migrated from a sibling cpu to a
> cpu on another core.
> 
> Fix this by using get_cpu()/ put_cpu()

Thanks.  Looks good and it's boots for me.

Signed-off-by: Michael Neuling <mikey@...ling.org>

> Signed-off-by: Shreyas B. Prabhu <shreyas@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/sysdev/xics/icp-native.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/sysdev/xics/icp-native.c b/arch/powerpc/sysdev/xics/icp-native.c
> index 2fc4cf1..62fd478 100644
> --- a/arch/powerpc/sysdev/xics/icp-native.c
> +++ b/arch/powerpc/sysdev/xics/icp-native.c
> @@ -147,12 +147,16 @@ static void icp_native_cause_ipi(int cpu, unsigned long data)
>  {
>  	kvmppc_set_host_ipi(cpu, 1);
>  #ifdef CONFIG_PPC_DOORBELL
> -	if (cpu_has_feature(CPU_FTR_DBELL) &&
> -	    (cpumask_test_cpu(cpu, cpu_sibling_mask(smp_processor_id()))))
> -		doorbell_cause_ipi(cpu, data);
> -	else
> +	if (cpu_has_feature(CPU_FTR_DBELL)) {
> +		if (cpumask_test_cpu(cpu, cpu_sibling_mask(get_cpu()))) {
> +			doorbell_cause_ipi(cpu, data);
> +			put_cpu();
> +			return;
> +		}
> +		put_cpu();
> +	}
>  #endif
> -		icp_native_set_qirr(cpu, IPI_PRIORITY);
> +	icp_native_set_qirr(cpu, IPI_PRIORITY);
>  }
>  
>  /*

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