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Message-ID: <bce79497-52c5-4241-aaf6-2a95dc459041@joelfernandes.org>
Date: Fri, 8 Mar 2024 09:57:42 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Stefan Wiehler <stefan.wiehler@...ia.com>,
Russell King <linux@...linux.org.uk>, "Paul E. McKenney"
<paulmck@...nel.org>, Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
linux-arm-kernel@...ts.infradead.org, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm: smp: Avoid false positive CPU hotplug Lockdep-RCU
splat
On 3/7/2024 11:09 AM, 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@...ia.com>
> ---
> arch/arm/kernel/smp.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 3431c0553f45..6875e2c5dd50 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -319,7 +319,14 @@ void __noreturn arch_cpu_idle_dead(void)
> {
> unsigned int cpu = smp_processor_id();
>
> + /*
> + * Briefly report CPU as online again to avoid false positive
> + * Lockdep-RCU splat when check_and_switch_context() acquires ASID
> + * spinlock.
> + */
> + rcutree_report_cpu_starting(cpu);
> idle_task_exit();
> + rcutree_report_cpu_dead();
I agree with the problem but disagree with the patch because it feels like a
terrible workaround.
Can we just use arch_spin_lock() for the cpu_asid_lock? This might require
acquiring the raw_lock within the raw_spinlock_t, but there is precedent:
arch/powerpc/kvm/book3s_hv_rm_mmu.c:245:
arch_spin_lock(&kvm->mmu_lock.rlock.raw_lock);
IMO, lockdep tracking of this lock is not necessary or possible considering the
hotplug situation.
Or is there a reason you need lockdep working for the cpu_asid_lock?
thanks,
- Joel
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