linux-kernel - Re: [PATCH 1/2] kernel/sofirq: consolidate common code in __tasklet_schedule() + _hi

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Message-ID: <20180215150707.49cc2332@gandalf.local.home>
Date:   Thu, 15 Feb 2018 15:07:07 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 1/2] kernel/sofirq: consolidate common code in
 __tasklet_schedule() + _hi_

On Thu, 15 Feb 2018 18:20:41 +0100
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> -void __tasklet_schedule(struct tasklet_struct *t)
> +static void __tasklet_schedule_common(struct tasklet_struct *t,
> +				      struct tasklet_head *head,
> +				      unsigned int softirq_nr)
>  {
>  	unsigned long flags;
>  
>  	local_irq_save(flags);

If you look at the original patch, it did not move local_irq_save()
into the common function.

>  	t->next = NULL;
> -	*__this_cpu_read(tasklet_vec.tail) = t;
> -	__this_cpu_write(tasklet_vec.tail, &(t->next));
> -	raise_softirq_irqoff(TASKLET_SOFTIRQ);
> +	*head->tail = t;
> +	head->tail = &(t->next);
> +	raise_softirq_irqoff(softirq_nr);
>  	local_irq_restore(flags);
>  }
> +
> +void __tasklet_schedule(struct tasklet_struct *t)
> +{
> +	__tasklet_schedule_common(t, this_cpu_ptr(&tasklet_vec),

What can happen is, we reference (tasklet_vec) on one CPU, get
preempted (running in ksoftirqd), scheduled on another CPU, then when
inside the common code, we are executing on a different CPU than the
tasklet is for. The rasise_softirq() is happening on the wrong CPU.

The local_irq_save() can't be moved to the common function. It must be
done by each individual function.

-- Steve


> +				  TASKLET_SOFTIRQ);
> +}
>  EXPORT_SYMBOL(__tasklet_schedule);