Re: [PATCH 09/19] arm64: smp: Defer RCU registration during secondary CPU bringup
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From: Will Deacon <will@kernel.org>
Date: 2026-09-09 12:37:10
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On Tue, Sep 08, 2026 at 07:25:54PM +0800, Jinjie Ruan wrote:
在 2026/9/8 18:19, Will Deacon 写道:quoted
On Tue, Sep 08, 2026 at 04:55:36PM +0800, Jinjie Ruan wrote:quoted
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