Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations

[Peter Zijlstra]

On Thu, Oct 10, 2024 at 08:32:07AM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-09 11:24:09 [-0700], Paul E. McKenney wrote:
> > In order to support systems that currently run CONFIG_PREEMPT=n that
> …
> > Or am I once again missing your point?
> 
> The change is:
> | config PREEMPT_RCU
> |        bool
> |-       default y if PREEMPTION
> |+       default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
> 
> Now:
> - CONFIG_PREEMPT select PREEMPT_BUILD
> - PREEMPT_RT select CONFIG_PREEMPTION
> - PREEMPT_DYNAMIC selects PREEMPT_BUILD
> 
> and PREEMPT_BUILD select CONFIG_PREEMPTION
> 
> so in the end, this change is a nop, right?

PREEMPT_RT selects PREEMPTION *and* has one of PREEMPT / PREEMPT_LAZY /
PREEMPT_DYNAMIC, all of which in turn select PREEMPT_BUILD, which
selects PREEMPTION.

(arguably we can remove the select PREEMPTION from PREEMPT_RT)

The proposed change is not a nop because the config: PREEMPT_LAZY=y
PREEMPT_DYNAMIC=n will result in false, while it will have PREEMPTION.

That said, I really do not agree with the change, it makes the condition
complicated for no reason.