Re: next-20211015: suspend to ram on x86-32 broken

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 18, 2021 at 01:44:29PM +0200, Pavel Machek wrote:

> Reverting smp.c hunk is enough to get suspend/resume to work.

Thanks! Queued the below.

---
Subject: sched: Partial revert: "sched: Simplify wake_up_*idle*()"
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon Oct 18 16:41:05 CEST 2021

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).

Therefore, undo this change and put in a comment :/

Fixes: 8850cb663b5c ("sched: Simplify wake_up_*idle*()")
Reported-by: syzbot+d5b23b18d2f4feae8a67@syzkaller.appspotmail.com
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Pavel Machek <pavel@ucw.cz>
---
 kernel/smp.c |   15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -1170,14 +1170,23 @@ void wake_up_all_idle_cpus(void)
 {
 	int cpu;
 
-	cpus_read_lock();
+	/*
+	 * This really should be cpus_read_lock(), because disabling preemption
+	 * over iterating all CPUs is really bad when you have large numbers of
+	 * CPUs, giving rise to large latencies.
+	 *
+	 * Sadly this cannot be, since (ironically) this function is used from
+	 * the cpu_latency_qos stuff which in turn is used under all sorts of
+	 * locks yielding a hotplug lock inversion :/
+	 */
+	preempt_disable();
 	for_each_online_cpu(cpu) {
-		if (cpu == raw_smp_processor_id())
+		if (cpu == smp_processor_id())
 			continue;
 
 		wake_up_if_idle(cpu);
 	}
-	cpus_read_unlock();
+	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);