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

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[ Upstream commit 6bb05a33337b2c842373857b63de5c9bf1ae2a09 ]

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: 7560c02bdf ("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: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Waiman Long 2025-01-31 12:33:23 -05:00 committed by Greg Kroah-Hartman
parent 0bebe3e832
commit 852805b6cb
1 changed files with 4 additions and 2 deletions
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6
kernel/time/clocksource.c
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@ -348,10 +348,10 @@ 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;
@ -359,6 +359,7 @@ 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;
@ -378,6 +379,7 @@ 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",