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Message-ID: <20250124185923.3610964-2-longman@redhat.com>
Date: Fri, 24 Jan 2025 13:59:23 -0500
From: Waiman Long <longman@...hat.com>
To: John Stultz <jstultz@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Stephen Boyd <sboyd@...nel.org>,
Feng Tang <feng.tang@...el.com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Clark Williams <clrkwllms@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org,
linux-rt-devel@...ts.linux.dev,
Waiman Long <longman@...hat.com>
Subject: [PATCH 2/2] clocksource: Defer preempt_disable() after clocksource_verify_choose_cpus()
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.
To avoid this problem, we can't call get_random_u32() with preemption
disabled. However, smp_processor_id() has to be called with preemption
disabled though and we have to exclude the current CPU from the
cpus_chosen list to be tested.
Extract current CPU removal code out from
clocksource_verify_choose_cpus() and defer the preempt_disable()
call to after clocksource_verify_choose_cpus() and before
current CPU removal. Also use raw_smp_processor_id() in
clocksource_verify_choose_cpus().
Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
Signed-off-by: Waiman Long <longman@...hat.com>
---
kernel/time/clocksource.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 77d9566d3aa6..8ffec49971ac 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -313,7 +313,6 @@ static void clocksource_verify_choose_cpus(void)
if (n < 0) {
/* Check all of the CPUs. */
cpumask_copy(&cpus_chosen, cpu_online_mask);
- cpumask_clear_cpu(smp_processor_id(), &cpus_chosen);
return;
}
@@ -322,9 +321,13 @@ static void clocksource_verify_choose_cpus(void)
if (n == 0 || num_online_cpus() <= 1)
return;
- /* Make sure to select at least one CPU other than the current CPU. */
+ /*
+ * Make sure to select at least one CPU other than the current CPU.
+ * raw_smp_processor_id() is used opportunistically here as
+ * preemption isn't disabled.
+ */
cpu = cpumask_first(cpu_online_mask);
- if (cpu == smp_processor_id())
+ if (cpu == raw_smp_processor_id())
cpu = cpumask_next(cpu, cpu_online_mask);
if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
return;
@@ -349,9 +352,6 @@ static void clocksource_verify_choose_cpus(void)
if (!WARN_ON_ONCE(cpu >= nr_cpu_ids))
cpumask_set_cpu(cpu, &cpus_chosen);
}
-
- /* Don't verify ourselves. */
- cpumask_clear_cpu(smp_processor_id(), &cpus_chosen);
}
static void clocksource_verify_one_cpu(void *csin)
@@ -373,15 +373,18 @@ void clocksource_verify_percpu(struct clocksource *cs)
cpumask_clear(&cpus_ahead);
cpumask_clear(&cpus_behind);
cpus_read_lock();
- preempt_disable();
clocksource_verify_choose_cpus();
+ preempt_disable();
+
+ /* Don't verify ourselves. */
+ testcpu = smp_processor_id();
+ cpumask_clear_cpu(testcpu, &cpus_chosen);
if (cpumask_empty(&cpus_chosen)) {
preempt_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));
for_each_cpu(cpu, &cpus_chosen) {
--
2.47.1
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