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Date:   Fri, 13 Jan 2023 11:38:17 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
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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