[question] will softirq handler potentially be called many times?

[question] will softirq handler potentially be called many times?

[Kai Huang]

Hi,

I see the __do_softirq is called when ! in_atomic(), which means
potentially __do_softirq may be interrupted by trap, exception,
interrupt, etc, so seems softirq handler may be executed many times?

For example, if interrupt happens after i =
find_first_set_bit(pending), the same softirq hander will be called
twice as the do_softriq will be called after all interrupt handler
returned, and the pending bit has not been cleared yet when first
do_softirq was called.

static void __do_softirq(unsigned long ignore_mask)
{
    ......
    for ( ; ; )
    {
        ......
        i = find_first_set_bit(pending);
                                                   <- interrupt happens
        clear_bit(i, &softirq_pending(cpu));
        (*softirq_handlers[i])();
    }
}

-cody

Re: [question] will softirq handler potentially be called many times?

[Keir Fraser]

On 17/02/2012 04:47, "Kai Huang" <mail.kai.huang@gmail.com> wrote:

> Hi,
> 
> I see the __do_softirq is called when ! in_atomic(), which means
> potentially __do_softirq may be interrupted by trap, exception,
> interrupt, etc, so seems softirq handler may be executed many times?
> 
> For example, if interrupt happens after i =
> find_first_set_bit(pending), the same softirq hander will be called
> twice as the do_softriq will be called after all interrupt handler
> returned, and the pending bit has not been cleared yet when first
> do_softirq was called.

Can't happen, we only call do_softirq() when returning from an interrupt
back to guest context. Therefore a nested interrupt call does not cause
do_softirq to run.

  -- Keir

> static void __do_softirq(unsigned long ignore_mask)
> {
>     ......
>     for ( ; ; )
>     {
>         ......
>         i = find_first_set_bit(pending);
>                                                    <- interrupt happens
>         clear_bit(i, &softirq_pending(cpu));
>         (*softirq_handlers[i])();
>     }
> }
> 
> -cody
> 
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