Re: Workqueues splat due to ending up on wrong CPU

[Peter Zijlstra]

On Wed, Dec 04, 2019 at 12:11:50PM -0800, Paul E. McKenney wrote:

> And the good news is that I didn't see the workqueue splat, though my
> best guess is that I had about a 13% chance of not seeing it due to
> random chance (and I am currently trying an idea that I hope will make
> it more probable).  But I did get a couple of new complaints about RCU
> being used illegally from an offline CPU.  Splats below.

Shiny!

> Your patch did rearrange the CPU-online sequence, so let's see if I
> can piece things together...
> 
> RCU considers a CPU to be online at rcu_cpu_starting() time.  This is
> called from notify_cpu_starting(), which is called from the arch-specific
> CPU-bringup code.  Any RCU readers before rcu_cpu_starting() will trigger
> the warning I am seeing.

Right.

> The original location of the stop_machine_unpark() was in
> bringup_wait_for_ap(), which is called from bringup_cpu(), which is in
> the CPUHP_BRINGUP_CPU entry of cpuhp_hp_states[].  Which, if I am not
> too confused, is invoked by some CPU other than the to-be-incoming CPU.

Correct.

> The new location of the stop_machine_unpark() is in cpuhp_online_idle(),
> which is called from cpu_startup_entry(), which is invoked from
> the arch-specific bringup code that runs on the incoming CPU.

The new place is the final piece of bringup, it is right before where
the freshly woken CPU will drop into the idle loop and start scheduling
(for the first time).

> Which
> is the same code that invokes notify_cpu_starting(), so we need
> notify_cpu_starting() to be invoked before cpu_startup_entry().

Right, that is right before we run what used to be the CPU_STARTING
notifiers. This is in fact (on x86) before the CPU is marked
cpu_online(). It has to be before cpu_startup_entry(), before this is
ran with IRQs disabled, while cpu_startup_entry() demands IRQs are
enabled.

> The order is not immediately obvious on IA64.  But it looks like
> everything else does it in the required order, so I am a bit confused
> about this.

That makes two of us, afaict we have RCU up and running when we get to
the idle loop.