From: Peter Zijlstra <peterz@infradead.org>
Date: Mon, 18 Oct 2021 14:41:05 +0000 (+0200)
Subject: sched: Improve wake_up_all_idle_cpus() take #2
X-Git-Tag: v5.16-rc1~197^2~8
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=96611c26dc351c33f73b48756a9feacc109e5bab;p=thirdparty%2Flinux.git

sched: Improve wake_up_all_idle_cpus() take #2

As reported by syzbot and experienced by Pavel, using cpus_read_lock()
in wake_up_all_idle_cpus() generates lock inversion (against mmap_sem
and possibly others).

Instead, shrink the preempt disable region by iterating all CPUs and
checking the online status for each individual CPU while having
preemption disabled.

Fixes: 8850cb663b5c ("sched: Simplify wake_up_*idle*()")
Reported-by: syzbot+d5b23b18d2f4feae8a67@syzkaller.appspotmail.com
Reported-by: Pavel Machek <pavel@ucw.cz>
Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Qian Cai <quic_qiancai@quicinc.com>
---

diff --git a/kernel/smp.c b/kernel/smp.c
index ad0b68a3a3d36..01a7c1706a58b 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -1170,14 +1170,12 @@ void wake_up_all_idle_cpus(void)
 {
 	int cpu;
 
-	cpus_read_lock();
-	for_each_online_cpu(cpu) {
-		if (cpu == raw_smp_processor_id())
-			continue;
-
-		wake_up_if_idle(cpu);
+	for_each_possible_cpu(cpu) {
+		preempt_disable();
+		if (cpu != smp_processor_id() && cpu_online(cpu))
+			wake_up_if_idle(cpu);
+		preempt_enable();
 	}
-	cpus_read_unlock();
 }
 EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);