Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

[Alan Stern]

On Thu, Jan 12, 2023 at 01:57:16PM -0800, Paul E. McKenney wrote:

> I will risk sharing the intuition behind the rcu-order counting rule.
> 
> In the code, an RCU read-side critical section begins with rcu_read_lock()
> and ends with the matching rcu_read_unlock().  RCU read-side critical
> section may be nested, in which case RCU cares only about the outermost
> of the nested set.
> 
> An RCU grace period includes at least one moment in time during which
> each and every process/CPU/task/whatever is not within an RCU read-side
> critical section.

Strictly speaking, this is not right.  It should say: For each 
process/CPU/task/whatever, an RCU grace period includes at least one 
moment in time during which that process is not within an RCU read-side 
critical section.  There does not have to be any single moment during 
which no processes are executing a critical section.

For example, the following is acceptable:

CPU 0:               start of synchronize_rcu()......end
CPU 1:  rcu_lock().....................rcu_unlock()
CPU 2:                    rcu_lock().......................rcu_unlock()

Alan