[PATCH v2] rcu: Use IRQ_WORK_INIT_HARD for srcu's irq_work

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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>
---
v1…v2: https://lore.kernel.org/all/20260820110008.v2Tlr0_H@linutronix.de/
- Add a comment as suggested by Steven.

 kernel/rcu/srcutree.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 7c2f7cc131f7a..af9fe0165a800 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -218,7 +218,12 @@ 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);
+	/*
+	 * trace events started on the command line require SRCU before
+	 * the irq_work kthread starts. Since all it does is a simple
+	 * wakeup, having it as a hard irq, even on PREEMPT_RT is fine.
+	 */
+	ssp->srcu_sup->irq_work = IRQ_WORK_INIT_HARD(srcu_irq_work);
 	ssp->srcu_sup->sda_is_static = is_static;
 	if (!is_static) {
 		ssp->sda = alloc_percpu(struct srcu_data);
-- 
2.55.0