[PATCH v2 2/2] clocksource: Use get_random_bytes() in clocksource_verify_choose_cpus()

[Waiman Long <longman@redhat.com>]

The following bug report happened in a PREEMPT_RT kernel.

[   30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
[   30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
[   30.962673] preempt_count: 1, expected: 0
[   30.962676] RCU nest depth: 0, expected: 0
[   30.962680] 3 locks held by kwatchdog/2012:
[   30.962684]  #0: ffffffff8af2da60 (clocksource_mutex){+.+.}-{3:3}, at: clocksource_watchdog_kthread+0x13/0x50
[   30.967703]  #1: ffffffff8aa8d4d0 (cpu_hotplug_lock){++++}-{0:0}, at: clocksource_verify_percpu.part.0+0x5c/0x330
[   30.972774]  #2: ffff9fe02f5f33e0 ((batched_entropy_u32.lock)){+.+.}-{2:2}, at: get_random_u32+0x4f/0x110
[   30.977827] Preemption disabled at:
[   30.977830] [<ffffffff88c1fe56>] clocksource_verify_percpu.part.0+0x66/0x330
[   30.982837] CPU: 33 PID: 2012 Comm: kwatchdog Not tainted 5.14.0-503.23.1.el9_5.x86_64+rt-debug #1
[   30.982843] Hardware name: HPE ProLiant DL385 Gen10 Plus/ProLiant DL385 Gen10 Plus, BIOS A42 04/29/2021
[   30.982846] Call Trace:
[   30.982850]  <TASK>
[   30.983821]  dump_stack_lvl+0x57/0x81
[   30.983821]  __might_resched.cold+0xf4/0x12f
[   30.983824]  rt_spin_lock+0x4c/0x100
[   30.988833]  get_random_u32+0x4f/0x110
[   30.988833]  clocksource_verify_choose_cpus+0xab/0x1a0
[   30.988833]  clocksource_verify_percpu.part.0+0x6b/0x330
[   30.993894]  __clocksource_watchdog_kthread+0x193/0x1a0
[   30.993898]  clocksource_watchdog_kthread+0x18/0x50
[   30.993898]  kthread+0x114/0x140
[   30.993898]  ret_from_fork+0x2c/0x50
[   31.002864]  </TASK>

It is due to the fact that get_random_u32() is called in
clocksource_verify_choose_cpus() with preemption disabled.
If crng_ready() is true by the time get_random_u32() is called, The
batched_entropy_32 local lock will be acquired. In PREEMPT_RT kernel,
it is a rtmutex and we can't acquire it with preemption disabled.

Fix this problem by using the less random get_random_bytes() function
which will not take any lock. In fact, it has the same random-ness as
get_random_u32_below() when crng_ready() is false.

Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/time/clocksource.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 77d9566d3aa6..659c4b79119c 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -340,9 +340,13 @@ static void clocksource_verify_choose_cpus(void)
 	 * and no replacement CPU is selected.  This gracefully handles
 	 * situations where verify_n_cpus is greater than the number of
 	 * CPUs that are currently online.
+	 *
+	 * The get_random_bytes() is used here to avoid taking lock with
+	 * preemption disabled.
 	 */
 	for (i = 1; i < n; i++) {
-		cpu = get_random_u32_below(nr_cpu_ids);
+		get_random_bytes(&cpu, sizeof(cpu));
+		cpu %= nr_cpu_ids;
 		cpu = cpumask_next(cpu - 1, cpu_online_mask);
 		if (cpu >= nr_cpu_ids)
 			cpu = cpumask_first(cpu_online_mask);
-- 
2.47.1