RE: [PATCH] softirq: add irq off checking for __raise_softirq_irqoff

[Jiafei Pan]

Any comments? Thanks.

@Steven Rostedt, I thinks irq off checking is necessary especially for Preempt-RT kernel, because some context may be changed from irq off to irq on when enable Preempt RT, I once met a issue that hrtimer soft irq is lost when enabled Preempt RT, finally I found napi_schedule_irqoff is called in hardware interrupt handler, there maybe no issue for non RT kernel, but for Preempt RT, interrupt is threaded, so irq is on in interrupt handler, the result is __raise_softirq_irqoff is called in irq on context, so that per-CPU softirq masking is corrupted because of the process of updating of soft irq masking is interrupted and not a atomic operation , and then caused hrtimer soft irq is lost. So I think adding irq status checking in __raise_softirq_irqoff can report such issue directly and help us to find the root cause of such issue.

I know that there may be performance impaction to add extra checking here, if it is the case, how about to include it in some debug configuration items? Such as CONFIG_DEBUG_PREEMPT or other debug items?

Best Regards,
Jiafei.

-----Original Message-----
From: Jiafei Pan <Jiafei.Pan@xxxxxxx> 
Sent: Thursday, August 6, 2020 12:07 PM
To: peterz@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; rostedt@xxxxxxxxxxx; romain.perier@xxxxxxxxx; will@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-rt-users@xxxxxxxxxxxxxxx; Jiafei Pan <jiafei.pan@xxxxxxx>; Leo Li <leoyang.li@xxxxxxx>; Vladimir Oltean <vladimir.oltean@xxxxxxx>; Jiafei Pan <jiafei.pan@xxxxxxx>
Subject: [PATCH] softirq: add irq off checking for __raise_softirq_irqoff

__raise_softirq_irqoff will update per-CPU mask of pending softirqs, it need to be called in irq disabled context in order to keep it atomic operation, otherwise it will be interrupted by hardware interrupt, and per-CPU softirqs pending mask will be corrupted, the result is there will be unexpected issue, for example hrtimer soft irq will be losed and soft hrtimer will never be expire and handled.

Adding irqs disabled checking here to provide warning in irqs enabled context.

Signed-off-by: Jiafei Pan <Jiafei.Pan@xxxxxxx>
---
 kernel/softirq.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/softirq.c b/kernel/softirq.c index bf88d7f62433..11f61e54a3ae 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -481,6 +481,11 @@ void raise_softirq(unsigned int nr)
 
 void __raise_softirq_irqoff(unsigned int nr)  {
+	/* This function can only be called in irq disabled context,
+	 * otherwise or_softirq_pending will be interrupted by hardware
+	 * interrupt, so that there will be unexpected issue.
+	 */
+	WARN_ON_ONCE(!irqs_disabled());
 	trace_softirq_raise(nr);
 	or_softirq_pending(1UL << nr);
 }
--
2.17.1