Re: [PATCH] arm: smp: Avoid false positive CPU hotplug Lockdep-RCU splat

[Russell King (Oracle)]

On Thu, Mar 07, 2024 at 05:09:51PM +0100, Stefan Wiehler wrote:
> With CONFIG_PROVE_RCU_LIST=y and by executing
> 
>   $ echo 0 > /sys/devices/system/cpu/cpu1/online
> 
> one can trigger the following Lockdep-RCU splat on ARM:
> 
>   =============================
>   WARNING: suspicious RCU usage
>   6.8.0-rc7-00001-g0db1d0ed8958 #10 Not tainted
>   -----------------------------
>   kernel/locking/lockdep.c:3762 RCU-list traversed in non-reader section!!
> 
>   other info that might help us debug this:
> 
>   RCU used illegally from offline CPU!
>   rcu_scheduler_active = 2, debug_locks = 1
>   no locks held by swapper/1/0.
> 
>   stack backtrace:
>   CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.8.0-rc7-00001-g0db1d0ed8958 #10
>   Hardware name: Allwinner sun8i Family
>    unwind_backtrace from show_stack+0x10/0x14
>    show_stack from dump_stack_lvl+0x60/0x90
>    dump_stack_lvl from lockdep_rcu_suspicious+0x150/0x1a0
>    lockdep_rcu_suspicious from __lock_acquire+0x11fc/0x29f8
>    __lock_acquire from lock_acquire+0x10c/0x348
>    lock_acquire from _raw_spin_lock_irqsave+0x50/0x6c
>    _raw_spin_lock_irqsave from check_and_switch_context+0x7c/0x4a8
>    check_and_switch_context from arch_cpu_idle_dead+0x10/0x7c
>    arch_cpu_idle_dead from do_idle+0xbc/0x138
>    do_idle from cpu_startup_entry+0x28/0x2c
>    cpu_startup_entry from secondary_start_kernel+0x11c/0x124
>    secondary_start_kernel from 0x401018a0
> 
> The CPU is already reported as offline from RCU perspective in
> cpuhp_report_idle_dead() before arch_cpu_idle_dead() is invoked. Above
> RCU-Lockdep splat is then triggered by check_and_switch_context() acquiring the
> ASID spinlock.
> 
> Avoid the false-positive Lockdep-RCU splat by briefly reporting the CPU as
> online again while the spinlock is held.
> 
> Signed-off-by: Stefan Wiehler <stefan.wiehler@xxxxxxxxx>

So what do I do about this? I see you submitted this to the patch system
on the 8th March, but proposed a different approach on the 9th March. I
don't see evidence that Paul is happy with the original approach either
and there's no ack/r-b's on anything here.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!