[PATCH 09/19] arm64: smp: Defer RCU registration during secondary CPU bringup

[Jinjie Ruan]

在 2026/9/8 18:19, Will Deacon 写道:
> Hi Jinjie,
> 
> On Tue, Sep 08, 2026 at 04:55:36PM +0800, Jinjie Ruan wrote:
>> 在 2026/9/8 0:40, Will Deacon 写道:
>>> Calling rcutree_report_cpu_starting() early during boot can lead to
>>> livelocks with the generic CPU hotplug mechanism if the boot CPU blocks
>>> on an RCU grace period while the CPU being onlined is spinning in
>>> cpuhp_ap_sync_alive().
>>>
>>> In preparation for enabling the generic CPU hotplug code on arm64, split
>>> up the trace_hardirqs_off() call during secondary CPU bringup so that we
>>> update lockdep early but defer the tracing updates until after
>>> notify_cpu_starting() has registered the new CPU with RCU, allowing us
>>> to drop the explicit call to rcutree_report_cpu_starting() entirely.
>>>
>>> Signed-off-by: Will Deacon <will at kernel.org>
>>> ---
>>>  arch/arm64/kernel/smp.c | 5 ++---
>>>  include/linux/rcutree.h | 2 +-
>>>  2 files changed, 3 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>>> index f4cabf9e19e6..ff68640d0c0b 100644
>>> --- a/arch/arm64/kernel/smp.c
>>> +++ b/arch/arm64/kernel/smp.c
>>> @@ -217,8 +217,7 @@ asmlinkage notrace void secondary_start_kernel(void)
>>>  	if (system_uses_irq_prio_masking())
>>>  		init_gic_priority_masking();
>>>  
>>> -	rcutree_report_cpu_starting(cpu);
>>> -	trace_hardirqs_off();
>>
>> I think we need to handle the printk problem before this patch as we
>> discussed earlier.
>>
>> Otherwise defer the rcutree_report_cpu_starting() will trigger a
>> false-positive lockdep"suspicious RCU usage" splat during early lock
>> acquisitions as commit ce3d31ad3cac ("arm64/smp: Move
>> rcu_cpu_starting() earlier") pointed out.
> 
> Sorry, I meant to mention this in the cover letter but forgot about it.
> I'm not sure that ce3d31ad3cac ("arm64/smp: Move rcu_cpu_starting()
> earlier") is still relevant with the latest printk/console/lockdep code.
> I tried quite hard to trigger lockdep splats manually, but the only way
> I could do it was by using the "%pS" specifier to print the name of a
> symbol in a module, which would cause an RCU walk of the module symbols
> in the kallsyms code! Manually calling WARN() or even rcu_read_lock() /
> spin_lock() did _not_ trigger a splat.

Hi Will,

Add "dyndbg="+p"" in cmdline, CONFIG_DEBUG_LOCK_ALLOC=y,
CONFIG_PROVE_RCU_LIST=y, we can reproduce the warning as below:

I believe there is also a problem in the RISC-V code itself here as
store_cpu_topology() is common for RISC-V.

[    0.335162] smp: Bringing up secondary CPUs ...
[    0.345495]
[    0.345513] =============================
[    0.345523] WARNING: suspicious RCU usage
[    0.345621] 7.3.0-rc2-00010-g2311ba2cd56f #500 Tainted: G        W
[    0.345637] -----------------------------
[    0.345646] kernel/locking/lockdep.c:3845 RCU-list traversed in
non-reader section!!
[    0.345659]
[    0.345659] other info that might help us debug this:
[    0.345659]
[    0.345680]
[    0.345680] RCU used illegally from offline CPU!
[    0.345680] rcu_scheduler_active = 1, debug_locks = 1
[    0.345725] locks held by swapper/1/0: 0, last CPU#1
[    0.345743]
[    0.345743] stack backtrace:
[    0.345834] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Tainted: G        W
         7.3.0-rc2-00010-g2311ba2cd56f #500 PREEMPT(full)
[    0.345885] Tainted: [W]=WARN
[    0.346077] Call trace:
[    0.346102]  show_stack+0x20/0x38 (C)
[    0.346153]  dump_stack_lvl+0xc4/0x150
[    0.346176]  dump_stack+0x18/0x28
[    0.346194]  lockdep_rcu_suspicious+0x170/0x238
[    0.346217]  __lock_acquire+0xf08/0x1818
[    0.346237]  lock_acquire+0x1e0/0x450
[    0.346256]  _raw_spin_lock_irqsave+0x70/0xc0
[    0.346277]  down_trylock+0x20/0x60
[    0.346293]  __down_trylock_console_sem+0x4c/0x118
[    0.346316]  vprintk_emit+0x2d8/0x3f8
[    0.346333]  vprintk_default+0x40/0x58
[    0.346350]  vprintk+0x3c/0x80
[    0.346366]  _printk+0x64/0x98
[    0.346386]  __dynamic_pr_debug+0x90/0xd8
[    0.346406]  acpi_get_cache_info+0x140/0x1a0
[    0.346430]  init_cache_level+0xec/0x110
[    0.346450]  detect_cache_attributes+0x74/0x7c0
[    0.346473]  update_siblings_masks+0x30/0x300
[    0.346495]  store_cpu_topology+0x70/0xf0
[    0.346515]  secondary_start_kernel+0xe0/0x178
[    0.346535]  __secondary_switched+0xc0/0xc8


> 
> Since that's not something I think we should be doing this early, I
> decided to leave the code as-is unless I have a way to trigger a lockdep
> splat with the relatively simple prints we have on the early error paths.
> 
> Will