Re: [PATCH 2/2] rcu: Robustify rcu_is_cpu_rrupt_from_idle()

[Frederic Weisbecker]

Le Sat, Mar 22, 2025 at 06:00:13PM +0100, Joel Fernandes a écrit :
> 
> 
> On 3/18/2025 2:56 PM, Frederic Weisbecker wrote:
> > RCU relies on the context tracking nesting counter in order to determine
> > if it is running in extended quiescent state.
> > 
> > However the context tracking nesting counter is not completely
> > synchronized with the actual context tracking state:
> > 
> > * The nesting counter is set to 1 or incremented further _after_ the
> >   actual state is set to RCU not watching.
> 
> I agree with patch, but this line is a bit confusing ->nesting is set to 1
> *after* the RCU state is set to "watching".  Did you mean "watching" ?
> 
> But I think you meant "After RCU transitions from a state of not-watching to
> watching' instead of 'actual state is set to RCU not watching'..
> 
> ct_kernel_entry():
> 
> 	// RCU is not watching here ...
> 	ct_kernel_enter_state(offset);
> 	// ... but is watching here.
> 	WRITE_ONCE(ct->nesting, 1);

Oh I completely inverted the thing in the changelog!

> 
> >    (then we know for sure we interrupted RCU not watching)
> > 
> > * The nesting counter is set to 0 or decremented further _before_ the
> >   actual state is set to RCU watching.
> > 
> > Therefore it is safe to assume that if ct_nesting() > 0, RCU is not
> > watching. But if ct_nesting() <= 0, RCU is watching except for a tiny
> > window.
> > 
> > This hasn't been a problem so far because rcu_is_cpu_rrupt_from_idle()
> > has only been called from interrupts. However the code is confusing
> 
> Agreed, and I could also see the existing code's snippet:
> 	WARN_ON_ONCE(!nesting && !is_idle_task(current));
> 
> .. not working if this function were to be called from non-interrupt kernel
> context.

Right.

I'll reissue that one.

Thanks!