This is a note to let you know that I've just added the patch titled
clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context
to the 6.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
clocksource-use-migrate_disable-to-avoid-calling-get.patch
and it can be found in the queue-6.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 260b25a3327f0749a6dde43fe624cd790fde8b01
Author: Waiman Long <longman@xxxxxxxxxx>
Date: Fri Jan 31 12:33:23 2025 -0500
clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context
[ 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: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20250131173323.891943-2-longman@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index c4e6b5e6af88c..58fb7280cabbe 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -365,10 +365,10 @@ 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;
@@ -376,6 +376,7 @@ 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;
@@ -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",
