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

[Paul E. McKenney]

On Thu, Mar 07, 2024 at 09:45:36AM -0800, Paul E. McKenney wrote:
> 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>
> 
> From an RCU perspective, this looks plausible.  One question
> below.

But one additional caution...  If execution is delayed during that call
to idle_task_exit(), RCU will stall and won't have a reasonable way of
motivating this CPU.  Such delays could be due to vCPU preemption or
due to firmware grabbing the CPU.

But this is only a caution, not opposition.  After all, you could have
the same problem with an online CPU that gets similarly delayed while
its interrupts are disabled.

						Thanx, Paul

> > ---
> >  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();
> >  
> >  	local_irq_disable();
> 
> Both rcutree_report_cpu_starting() and rcutree_report_cpu_dead() complain
> bitterly via lockdep if interrupts are enabled.  And the call sites have
> interrupts disabled.  So I don't understand what this local_irq_disable()
> is needed for.