linux-kernel - Re: [PATCH] irqchip/gic-v3: use dsb(ishst) to synchronize data to smp before issuing ipi

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Date:   Sun, 20 Feb 2022 15:04:18 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Barry Song <21cnbao@...il.com>
Cc:     maz@...nel.org, tglx@...utronix.de, will@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxarm@...wei.com, Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH] irqchip/gic-v3: use dsb(ishst) to synchronize data to
 smp before issuing ipi

On Sat, Feb 19, 2022 at 05:55:49AM +0800, Barry Song wrote:
> dsb(ishst) should be enough here as we only need to guarantee the
> visibility of data to other CPUs in smp inner domain before we
> send the ipi.
> 
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> ---
>  drivers/irqchip/irq-gic-v3.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index 5e935d97207d..0efe1a9a9f3b 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -1211,7 +1211,7 @@ static void gic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
>  	 * Ensure that stores to Normal memory are visible to the
>  	 * other CPUs before issuing the IPI.
>  	 */
> -	wmb();
> +	dsb(ishst);

On ARM, wmb() is a dsb(st) followed by other operations which may
include a sync operation at the L2 cache, and SoC specific barriers
for the bus. Hopefully, nothing will break if the sledge hammer is
replaced by the tack hammer.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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