Re: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2015-04-16 at 18:28 +0200, Jan Kiszka wrote:
> Instead of turning all irq_work requests into lazy ones on -rt, just
> move their execution from hard into soft-irq context.
> 
> This resolves deadlocks of ftrace which will queue work from 
> arbitrary
> contexts, including those that have locks held that are needed for
> raising a soft-irq.

Yup, trace-cmd record -> dead-box fully repeatable, and now fixed.

> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> 
> Second try, looks much better so far. And it also removes my concerns
> regarding other potential cases besides ftrace.
> 
>  kernel/irq_work.c | 26 ++++++++++----------------
>  1 file changed, 10 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 9dda38a..3f6ffcd 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -85,12 +85,9 @@ bool irq_work_queue_on(struct irq_work *work, int 
> cpu)
>               raise_irqwork = llist_add(&work->llnode,
>                                         &per_cpu(hirq_work_list, cpu));
>       else
> -             raise_irqwork = llist_add(&work->llnode,
> -                                       &per_cpu(lazy_list, cpu));
> -#else
> +#endif
>               raise_irqwork = llist_add(&work->llnode,
>                                         &per_cpu(raised_list, cpu));
> -#endif
>  
>       if (raise_irqwork)
>               arch_send_call_function_single_ipi(cpu);
> @@ -114,21 +111,20 @@ bool irq_work_queue(struct irq_work *work)
>       if (work->flags & IRQ_WORK_HARD_IRQ) {
>               if (llist_add(&work->llnode, this_cpu_ptr(&hirq_work_list)))
>                       arch_irq_work_raise();
> -     } else {
> +     } else
> +#endif
> +     if (work->flags & IRQ_WORK_LAZY) {
>               if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
>                   tick_nohz_tick_stopped())
> +#ifdef CONFIG_PREEMPT_RT_FULL
>                       raise_softirq(TIMER_SOFTIRQ);
> -     }
>  #else
> -     if (work->flags & IRQ_WORK_LAZY) {
> -             if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
> -                 tick_nohz_tick_stopped())
>                       arch_irq_work_raise();
> +#endif
>       } else {
>               if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
>                       arch_irq_work_raise();
>       }
> -#endif
>  
>       preempt_enable();
>  
> @@ -202,6 +198,8 @@ void irq_work_run(void)
>  {
>  #ifdef CONFIG_PREEMPT_RT_FULL
>       irq_work_run_list(this_cpu_ptr(&hirq_work_list));
> +     if (!llist_empty(this_cpu_ptr(&raised_list)))
> +             raise_softirq(TIMER_SOFTIRQ);
>  #else
>       irq_work_run_list(this_cpu_ptr(&raised_list));
>       irq_work_run_list(this_cpu_ptr(&lazy_list));
> @@ -211,15 +209,11 @@ EXPORT_SYMBOL_GPL(irq_work_run);
>  
>  void irq_work_tick(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT_FULL
> -     irq_work_run_list(this_cpu_ptr(&lazy_list));
> -#else
> -     struct llist_head *raised = &__get_cpu_var(raised_list);
> +     struct llist_head *raised = this_cpu_ptr(&raised_list);
>  
>       if (!llist_empty(raised) && !arch_irq_work_has_interrupt())
>               irq_work_run_list(raised);
> -     irq_work_run_list(&__get_cpu_var(lazy_list));
> -#endif
> +     irq_work_run_list(this_cpu_ptr(&lazy_list));
>  }
>  
>  /*