linux-kernel - Re: [syzbot] [kernel?] WARNING: suspicious RCU usage in __do

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Date: Mon, 22 Apr 2024 23:15:28 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Z qiang <qiang.zhang1211@...il.com>, paulmck@...nel.org
Cc: syzbot <syzbot+dce04ed6d1438ad69656@...kaller.appspotmail.com>,
 linux-kernel@...r.kernel.org, peterz@...radead.org,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kernel?] WARNING: suspicious RCU usage in __do_softirq

On Fri, Apr 19 2024 at 13:50, Z qiang wrote:
>> On Thu, Apr 18, 2024 at 05:49:38PM +0800, Z qiang wrote:
>  static __init int spawn_ksoftirqd(void)
>  {
> +       int cpu;
> +
> +       for_each_possible_cpu(cpu)
> +               per_cpu(ksoftirqd_work, cpu) = false;

First of all that initialization is pointless, but why making all of
this complex as hell?

Thanks,

        tglx
---
diff --git a/kernel/softirq.c b/kernel/softirq.c
index b315b21fb28c..e991d735be0d 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -508,7 +508,7 @@ static inline bool lockdep_softirq_start(void) { return false; }
 static inline void lockdep_softirq_end(bool in_hardirq) { }
 #endif
 
-asmlinkage __visible void __softirq_entry __do_softirq(void)
+static void handle_softirqs(bool kirqd)
 {
 	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 	unsigned long old_flags = current->flags;
@@ -563,8 +563,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 		pending >>= softirq_bit;
 	}
 
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
-	    __this_cpu_read(ksoftirqd) == current)
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kirqd)
 		rcu_softirq_qs();
 
 	local_irq_disable();
@@ -584,6 +583,11 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 	current_restore_flags(old_flags, PF_MEMALLOC);
 }
 
+asmlinkage __visible void __softirq_entry __do_softirq(void)
+{
+	handle_softirqs(false);
+}
+
 /**
  * irq_enter_rcu - Enter an interrupt context with RCU watching
  */
@@ -921,7 +925,7 @@ static void run_ksoftirqd(unsigned int cpu)
 		 * We can safely run softirq on inline stack, as we are not deep
 		 * in the task stack here.
 		 */
-		__do_softirq();
+		handle_softirqs(true);
 		ksoftirqd_run_end();
 		cond_resched();
 		return;