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

[Will Deacon]

On Tue, Sep 08, 2026 at 07:25:54PM +0800, Jinjie Ruan wrote:
> 在 2026/9/8 18:19, Will Deacon 写道:
> > On Tue, Sep 08, 2026 at 04:55:36PM +0800, Jinjie Ruan wrote:
> >> 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.
> 
> 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

I was about to say "don't do this" but then I realised two things:

1. update_siblings_masks() can trigger lockdep splats outside of
   pr_debug() if RCU isn't up and running, e.g.:

    [    0.524042]  show_stack+0x18/0x24 (C)
    [    0.524519]  __dump_stack+0x28/0x38
    [    0.524546]  dump_stack_lvl+0x64/0x84
    [    0.524562]  dump_stack+0x18/0x24
    [    0.524576]  lockdep_rcu_suspicious+0x134/0x1cc
    [    0.524591]  __lock_acquire+0xee8/0x2cb0
    [    0.524606]  lock_acquire+0x11c/0x2fc
    [    0.524621]  _raw_spin_lock_irqsave+0x64/0x84
    [    0.524641]  of_find_property+0x2c/0x8c
    [    0.524659]  detect_cache_attributes+0x1c0/0x6d0
    [    0.524676]  update_siblings_masks+0x38/0x288
    [    0.524692]  store_cpu_topology+0x4c/0x58
    [    0.524706]  secondary_start_kernel+0xdc/0x1c8
    [    0.524722]  __secondary_switched+0x120/0x124

2. This code is running _after_ cpuhp_ap_sync_alive().

So for the next version, I'll reintroduce the call to
rcutree_report_cpu_starting(), but move it immediately after the call to
cpuhp_ap_sync_alive(). I think that will solve these issues, without
causing issues with the concurrent part of early boot and also without
reintroducing the early call to rcutree_report_cpu_dead().

Cheers,

Will