Re: [PATCH] rcu: Use IRQ_WORK_INIT_HARD for srcu's irq_work

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Aug 20, 2026 at 01:00:08PM +0200, Sebastian Andrzej Siewior a écrit :
> The irq_work in srcu is used to schedule a delayed work. The work item
> is not scheduled directly because it is not always possible wake a
> thread directly.
> 
> On PREEMPT_RT the default irq_work is initialized with IRQ_WORK_LAZY and
> is delayed to the irq_work thread. A system booted with the command line
> "trace_event=…" will freeze during boot because the irq_work thread is
> not yet deployed (and tracing uses synchronize_srcu() in
> tp_rcu_cond_sync()).
> 
> The irq_work performs just a wakeup a thread, there is nothing wrong
> with doing this from hardirq context on PREEMPT_RT.
> 
> Use IRQ_WORK_INIT_HARD for srcu's irq_work.
> 
> Fixes: 7c405fb3279b3 ("rcu: Use an intermediate irq_work to start process_srcu()")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  kernel/rcu/srcutree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 7c2f7cc131f7a..b703233d4a63f 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -218,7 +218,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
>  	mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);
>  	atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
>  	INIT_DELAYED_WORK(&ssp->srcu_sup->work, process_srcu);
> -	init_irq_work(&ssp->srcu_sup->irq_work, srcu_irq_work);
> +	ssp->srcu_sup->irq_work = IRQ_WORK_INIT_HARD(srcu_irq_work);

It may need a comment to not lose that.

>  	ssp->srcu_sup->sda_is_static = is_static;
>  	if (!is_static) {
>  		ssp->sda = alloc_percpu(struct srcu_data);
> -- 
> 2.55.0
> 
>

Perhaps all irq works queued before the relevant kthread is started should
temporarily be queued as hard?

Thanks.

-- 
Frederic Weisbecker
SUSE Labs