linux-kernel - Re: [PATCH] irqchip/gic-v4: Fix ordering between vmapp and vpe locks

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Message-ID: <875xsrvpt3.ffs@tglx>
Date: Fri, 26 Jul 2024 22:52:40 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: Zhou Wang <wangzhou1@...ilicon.com>
Subject: Re: [PATCH] irqchip/gic-v4: Fix ordering between vmapp and vpe locks

On Tue, Jul 23 2024 at 18:52, Marc Zyngier wrote:
> @@ -3808,7 +3802,7 @@ static int its_vpe_set_affinity(struct irq_data *d,
>  	struct its_vpe *vpe = irq_data_get_irq_chip_data(d);
>  	unsigned int from, cpu = nr_cpu_ids;
>  	struct cpumask *table_mask;
> -	unsigned long flags;
> +	unsigned long flags, vmapp_flags;

What's this flags business for? its_vpe_set_affinity() is called with
interrupts disabled, no?
  
>  	/*
>  	 * Changing affinity is mega expensive, so let's be as lazy as
> @@ -3822,7 +3816,14 @@ static int its_vpe_set_affinity(struct irq_data *d,
>  	 * protect us, and that we must ensure nobody samples vpe->col_idx
>  	 * during the update, hence the lock below which must also be
>  	 * taken on any vLPI handling path that evaluates vpe->col_idx.
> +	 *
> +	 * Finally, we must protect ourselves against concurrent
> +	 * updates of the mapping state on this VM should the ITS list
> +	 * be in use.
>  	 */
> +	if (its_list_map)
> +		raw_spin_lock_irqsave(&vpe->its_vm->vmapp_lock, vmapp_flags);

Confused. This changes the locking from unconditional to
conditional. What's the rationale here?

Thanks,

        tglx