Re: [PATCH] ring-buffer: Use a housekeeping CPU to wake up waiters

On Wed, 7 Jan 2026 10:51:37 +0100
Petr Tesarik [off-list ref] wrote:

Erm. It's actually good I had a look. :-(

A helpful comment in irq_work_queue_on() explains that "arch remote IPI
send/receive backend aren't NMI safe". That's something I wasn't aware
of, and I'm afraid it's the end of story. The comment is followed by a
WARN_ON_ONCE(in_nmi()), and I can easily trigger it with "perf top"
while nmi:nmi_handler is traced.

Please, remove the patch again. I'm sorry.

Or we simply change it to:

static inline void
rb_irq_work_queue(struct rb_irq_work *irq_work)
{
	int cpu;

	/* irq_work_queue_on() is not allowed in NMI context */
	if (in_nmi()) {
		irq_work_queue(&irq_work->work, cpu);
		return;
	}

	cpu = housekeeping_any_cpu(HK_TYPE_KERNEL_NOISE);
	/*
	 * If CPU isolation is not active, cpu is always the current
	 * CPU, and the following is equivallent to irq_work_queue().
	 */
	irq_work_queue_on(&irq_work->work, cpu);
}


-- Steve