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

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Message-ID: <Y8GI+dbJ3uadVzSb@rowland.harvard.edu>
Date:   Fri, 13 Jan 2023 11:38:17 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "parri.andrea" <parri.andrea@...il.com>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

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