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@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

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 77d9566d3aa68cfa9af9285530becd959765525c..2a7802ec480cc5d0ea03d1b4666d321f196eeb56 100644 (file)
--- 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",