The following commit has been merged into the timers/urgent branch of tip:

Commit-ID: 01cfc84024e9a6b619696a35d2e5662255001cd0 Gitweb: https://git.kernel.org/tip/01cfc84024e9a6b619696a35d2e5662255001cd0 Author: Waiman Long longman@redhat.com AuthorDate: Fri, 24 Jan 2025 20:54:42 -05:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Mon, 27 Jan 2025 10:30:59 +01:00

clocksource: Use get_random_bytes() in clocksource_verify_choose_cpus()

The following bug report happened in 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 3 locks held by kwatchdog/2012: #0: ffffffff8af2da60 (clocksource_mutex){+.+.}-{3:3}, at: clocksource_watchdog_kthread+0x13/0x50 #1: ffffffff8aa8d4d0 (cpu_hotplug_lock){++++}-{0:0}, at: clocksource_verify_percpu.part.0+0x5c/0x330 #2: ffff9fe02f5f33e0 ((batched_entropy_u32.lock)){+.+.}-{2:2}, at: get_random_u32+0x4f/0x110 Preemption disabled at: [<ffffffff88c1fe56>] clocksource_verify_percpu.part.0+0x66/0x330 CPU: 33 PID: 2012 Comm: kwatchdog Not tainted 5.14.0-503.23.1.el9_5.x86_64+rt-debug #1 Call Trace: <TASK> __might_resched.cold+0xf4/0x12f rt_spin_lock+0x4c/0x100 get_random_u32+0x4f/0x110 clocksource_verify_choose_cpus+0xab/0x1a0 clocksource_verify_percpu.part.0+0x6b/0x330 __clocksource_watchdog_kthread+0x193/0x1a0 clocksource_watchdog_kthread+0x18/0x50 kthread+0x114/0x140 ret_from_fork+0x2c/0x50 </TASK>

This happens 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 a PREEMPT_RT enabled kernel, it is a rtmutex, which can't be acquireq 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 Signed-off-by: Thomas Gleixner tglx@linutronix.de Suggested-by: Paul E. McKenney paulmck@kernel.org Reviewed-by: Paul E. McKenney paulmck@kernel.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/all/20250125015442.3740588-2-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 77d9566..659c4b7 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);