Re: [PATCH] powerpc/smp: Move rcu_cpu_starting() earlier

From: Michael Ellerman
Date: Wed Oct 28 2020 - 20:09:48 EST


Qian Cai <cai@xxxxxxxxxx> writes:
> The call to rcu_cpu_starting() in start_secondary() is not early enough
> in the CPU-hotplug onlining process, which results in lockdep splats as
> follows:

Since when?
What kernel version?

I haven't seen this running CPU hotplug tests with PROVE_LOCKING=y on
v5.10-rc1. Am I missing a CONFIG?

cheers


> WARNING: suspicious RCU usage
> -----------------------------
> kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
>
> other info that might help us debug this:
>
> RCU used illegally from offline CPU!
> rcu_scheduler_active = 1, debug_locks = 1
> no locks held by swapper/1/0.
>
> Call Trace:
> dump_stack+0xec/0x144 (unreliable)
> lockdep_rcu_suspicious+0x128/0x14c
> __lock_acquire+0x1060/0x1c60
> lock_acquire+0x140/0x5f0
> _raw_spin_lock_irqsave+0x64/0xb0
> clockevents_register_device+0x74/0x270
> register_decrementer_clockevent+0x94/0x110
> start_secondary+0x134/0x800
> start_secondary_prolog+0x10/0x14
>
> This is avoided by moving the call to rcu_cpu_starting up near the
> beginning of the start_secondary() function. Note that the
> raw_smp_processor_id() is required in order to avoid calling into
> lockdep before RCU has declared the CPU to be watched for readers.
>
> Link: https://lore.kernel.org/lkml/160223032121.7002.1269740091547117869.tip-bot2@tip-bot2/
> Signed-off-by: Qian Cai <cai@xxxxxxxxxx>
> ---
> arch/powerpc/kernel/smp.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 3c6b9822f978..8c2857cbd960 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1393,13 +1393,14 @@ static void add_cpu_to_masks(int cpu)
> /* Activate a secondary processor. */
> void start_secondary(void *unused)
> {
> - unsigned int cpu = smp_processor_id();
> + unsigned int cpu = raw_smp_processor_id();
>
> mmgrab(&init_mm);
> current->active_mm = &init_mm;
>
> smp_store_cpu_info(cpu);
> set_dec(tb_ticks_per_jiffy);
> + rcu_cpu_starting(cpu);
> preempt_disable();
> cpu_callin_map[cpu] = 1;
>
> --
> 2.28.0