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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc:     linux-kernel@vger.kernel.org, kernel@axis.com,
        Vincent Whitchurch <vincent.whitchurch@axis.com>
Subject: Re: [PATCH v2] genirq: Fix nested thread vs synchronize_hardirq()
 deadlock
In-Reply-To: <20230613-genirq-nested-v2-1-fd5114b1e90c@axis.com>
References: <20230613-genirq-nested-v2-1-fd5114b1e90c@axis.com>
Date:   Fri, 30 Jun 2023 11:07:08 +0200
Message-ID: <87leg1z5c3.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Tue, Jun 20 2023 at 13:16, Vincent Whitchurch wrote:
> --- a/kernel/irq/chip.c
> +++ b/kernel/irq/chip.c
> @@ -476,7 +476,7 @@ void handle_nested_irq(unsigned int irq)
>  	}
>  
>  	kstat_incr_irqs_this_cpu(desc);
> -	irqd_set(&desc->irq_data, IRQD_IRQ_INPROGRESS);
> +	atomic_inc(&desc->threads_active);
>  	raw_spin_unlock_irq(&desc->lock);
>  
>  	action_ret = IRQ_NONE;
> @@ -487,7 +487,8 @@ void handle_nested_irq(unsigned int irq)
>  		note_interrupt(desc, action_ret);
>  
>  	raw_spin_lock_irq(&desc->lock);
> -	irqd_clear(&desc->irq_data, IRQD_IRQ_INPROGRESS);
> +	if (atomic_dec_and_test(&desc->threads_active))
> +		wake_up(&desc->wait_for_threads);

This breaks on RT. The wakeup cannot be inside the raw spin-locked
region.

Also this is open coding wake_threads_waitq().
  
> +static void __synchronize_irq(struct irq_desc *desc)
> +{
> +	__synchronize_hardirq(desc, true);
> +	/*
> +	 * We made sure that no hardirq handler is
> +	 * running. Now verify that no threaded handlers are
> +	 * active.
> +	 */
> +	wait_event(desc->wait_for_threads,
> +		   !atomic_read(&desc->threads_active));

Splitting this out is fine. Not reformatting it not so much.

Thanks,

        tglx