linux-kernel - Re: [PATCH v3] irq/core: synchronize irq

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Message-ID: <877d73n3eq.ffs@tglx>
Date:   Mon, 02 May 2022 21:24:45 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Thomas Pfaff <tpfaff@....com>, linux-kernel@...r.kernel.org,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v3] irq/core: synchronize irq_thread startup

On Tue, May 03 2022 at 00:01, Hillf Danton wrote:
> On Mon, 02 May 2022 16:28:56 +0200 Thomas Gleixner  wrote:
> --- upstream/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -1258,6 +1258,7 @@ static int irq_thread(void *data)
>  	struct irq_desc *desc = irq_to_desc(action->irq);
>  	irqreturn_t (*handler_fn)(struct irq_desc *desc,
>  			struct irqaction *action);
> +	bool waked = false;
>  
>  	sched_set_fifo(current);
>  
> @@ -1282,8 +1283,11 @@ static int irq_thread(void *data)
>  			irq_wake_secondary(desc, action);
>  
>  		wake_threads_waitq(desc);
> +		waked = true;
>  	}
>  
> +	if (!waked)
> +		wake_threads_waitq(desc);

That's a guarantee to make desc->threads_active go negative in the case
that the thread was never woken by a hard interrupt handler. IOW, you
created a new problem which did not exist before.

The problem discussed here is not a problem in irq_thread(), it's a
problem of not reaching this function in the first place. See the on
point analysis in Thomas Pfaffs patch.

Thanks,

        tglx