linux-kernel - [tip: timers/urgent] tick/rcu: Fix false positive "softirq work is pending" messages

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Message-ID: <169339140347.27769.9753097427140824629.tip-bot2@tip-bot2>
Date:   Wed, 30 Aug 2023 10:30:03 -0000
From:   "tip-bot2 for Paul Gortmaker" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        Wen Yang <wenyang.linux@...mail.com>,
        Frederic Weisbecker <frederic@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: timers/urgent] tick/rcu: Fix false positive "softirq work is
 pending" messages

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     96c1fa04f089a7e977a44e4e8fdc92e81be20bef
Gitweb:        https://git.kernel.org/tip/96c1fa04f089a7e977a44e4e8fdc92e81be20bef
Author:        Paul Gortmaker <paul.gortmaker@...driver.com>
AuthorDate:    Fri, 18 Aug 2023 16:07:57 -04:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Wed, 30 Aug 2023 12:20:28 +02:00

tick/rcu: Fix false positive "softirq work is pending" messages

In commit 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle") the
new function report_idle_softirq() was created by breaking code out of the
existing can_stop_idle_tick() for kernels v5.18 and newer.

In doing so, the code essentially went from a one conditional:

	if (a && b && c)
		warn();

to a three conditional:

	if (!a)
		return;
	if (!b)
		return;
	if (!c)
		return;
	warn();

But that conversion got the condition for the RT specific
local_bh_blocked() wrong. The original condition was:

   	!local_bh_blocked()

but the conversion failed to negate it so it ended up as:

        if (!local_bh_blocked())
		return false;

This issue lay dormant until another fixup for the same commit was added
in commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition").
This commit realized the ratelimit was essentially set to zero instead
of ten, and hence *no* softirq pending messages would ever be issued.

Once this commit was backported via linux-stable, both the v6.1 and v6.4
preempt-rt kernels started printing out 10 instances of this at boot:

  NOHZ tick-stop error: local softirq work is pending, handler #80!!!

Remove the negation and return when local_bh_blocked() evaluates to true to
bring the correct behaviour back.

Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Ahmad Fatoum <a.fatoum@...gutronix.de>
Reviewed-by: Wen Yang <wenyang.linux@...mail.com>
Acked-by: Frederic Weisbecker <frederic@...nel.org>
Link: https://lore.kernel.org/r/20230818200757.1808398-1-paul.gortmaker@windriver.com

---
 kernel/time/tick-sched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 4df14db..87015e9 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1045,7 +1045,7 @@ static bool report_idle_softirq(void)
 		return false;

 	/* On RT, softirqs handling may be waiting on some lock */
-	if (!local_bh_blocked())
+	if (local_bh_blocked())
 		return false;

 	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",