linux-kernel - [tip: timers/urgent] clocksource: Use migrate_disable() to avoid calling get_random

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Message-ID: <173859669633.10177.1567684612873040628.tip-bot2@tip-bot2>
Date: Mon, 03 Feb 2025 15:31:36 -0000
From: "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Waiman Long <longman@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
 "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: timers/urgent] clocksource: Use migrate_disable() to avoid
 calling get_random_u32() in atomic context

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

Commit-ID:     6bb05a33337b2c842373857b63de5c9bf1ae2a09
Gitweb:        https://git.kernel.org/tip/6bb05a33337b2c842373857b63de5c9bf1ae2a09
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Fri, 31 Jan 2025 12:33:23 -05:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Mon, 03 Feb 2025 16:18:56 +01:00

clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

The following bug report happened with a PREEMPT_RT kernel:

  BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
  preempt_count: 1, expected: 0
  RCU nest depth: 0, expected: 0
  get_random_u32+0x4f/0x110
  clocksource_verify_choose_cpus+0xab/0x1a0
  clocksource_verify_percpu.part.0+0x6b/0x330
  clocksource_watchdog_kthread+0x193/0x1a0

It is due to the fact that clocksource_verify_choose_cpus() is invoked with
preemption disabled.  This function invokes get_random_u32() to obtain
random numbers for choosing CPUs.  The batched_entropy_32 local lock and/or
the base_crng.lock spinlock in driver/char/random.c will be acquired during
the call. In PREEMPT_RT kernel, they are both sleeping locks and so cannot
be acquired in atomic context.

Fix this problem by using migrate_disable() to allow smp_processor_id() to
be reliably used without introducing atomic context. preempt_disable() is
then called after clocksource_verify_choose_cpus() but before the
clocksource measurement is being run to avoid introducing unexpected
latency.

Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Waiman Long <longman@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Link: https://lore.kernel.org/all/20250131173323.891943-2-longman@redhat.com
---
 kernel/time/clocksource.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 77d9566..2a7802e 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -373,10 +373,10 @@ void clocksource_verify_percpu(struct clocksource *cs)
 	cpumask_clear(&cpus_ahead);
 	cpumask_clear(&cpus_behind);
 	cpus_read_lock();
-	preempt_disable();
+	migrate_disable();
 	clocksource_verify_choose_cpus();
 	if (cpumask_empty(&cpus_chosen)) {
-		preempt_enable();
+		migrate_enable();
 		cpus_read_unlock();
 		pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name);
 		return;
@@ -384,6 +384,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
 	testcpu = smp_processor_id();
 	pr_info("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n",
 		cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
+	preempt_disable();
 	for_each_cpu(cpu, &cpus_chosen) {
 		if (cpu == testcpu)
 			continue;
@@ -403,6 +404,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
 			cs_nsec_min = cs_nsec;
 	}
 	preempt_enable();
+	migrate_enable();
 	cpus_read_unlock();
 	if (!cpumask_empty(&cpus_ahead))
 		pr_warn("        CPUs %*pbl ahead of CPU %d for clocksource %s.\n",