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

JIRA: https://issues.redhat.com/browse/RHEL-77959
Upstream Status: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git

commit 6bb05a3
Author: Waiman Long <longman@redhat.com>
Date:   Fri, 31 Jan 2025 12:33:23 -0500

    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: 7560c02 ("clocksource: Check per-CPU clock synchronization when marked unstable")
    Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Waiman Long <longman@redhat.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
    Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Link: https://lore.kernel.org/all/20250131173323.891943-2-longman@redhat.com

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

kernel/time/clocksource.c

@@ -365,17 +365,18 @@ 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;
 	}
 	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;
@@ -395,6 +396,7 @@ 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",