linux-kernel - Re: [PATCH 3/4] soc: qcom: smp2p: Add memory barrier for irq

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Date:   Wed, 6 Jul 2022 18:22:14 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Chris Lew <quic_clew@...cinc.com>
Cc:     agross@...nel.org, bjorn.andersson@...aro.org,
        konrad.dybcio@...ainline.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] soc: qcom: smp2p: Add memory barrier for irq_pending

On Tue, Jul 05, 2022 at 11:02:10PM -0700, Chris Lew wrote:
> There is a very tight race where the irq_retrigger function is run
> on one cpu and the actual retrigger softirq is running on a second
> cpu. When this happens, there may be a chance that the second cpu
> will not see the updated irq_pending value from first cpu.
> 
> Add a memory barrier to ensure that irq_pending is read correctly.
> 
> Signed-off-by: Chris Lew <quic_clew@...cinc.com>
> ---
>  drivers/soc/qcom/smp2p.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/soc/qcom/smp2p.c b/drivers/soc/qcom/smp2p.c
> index a94cddcb0298..a1ea5f55c228 100644
> --- a/drivers/soc/qcom/smp2p.c
> +++ b/drivers/soc/qcom/smp2p.c
> @@ -249,6 +249,9 @@ static void qcom_smp2p_notify_in(struct qcom_smp2p *smp2p)
>  
>  		status = val ^ entry->last_value;
>  		entry->last_value = val;
> +
> +		/* Ensure irq_pending is read correctly */
> +		mb();

I don't quite understand why you need a barrier here. mb() makes sure all the
prior instructions gets executed before executing the later one. But why is it
needed here?

>  		status |= *entry->irq_pending;
>  
>  		/* No changes of this entry? */
> @@ -356,6 +359,11 @@ static int smp2p_retrigger_irq(struct irq_data *irqd)
>  
>  	set_bit(irq, entry->irq_pending);
>  
> +	/* Ensure irq_pending is visible to all cpus that retried interrupt
> +	 * can run on
> +	 */
> +	mb();
> +

Here it makes sense because you want the CPU to set irq_pending before exiting
from this function. But even then you can use the less strict smp_wmb() that
serves the exact purpose.

Thanks,
Mani

>  	return 0;
>  }
>  
> -- 
> 2.7.4
> 

-- 
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