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Date:   Sat, 14 Jan 2023 09:42:06 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Jonas Oberhauser <jonas.oberhauser@huawei.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Peter Zijlstra <peterz@infradead.org>,
        "parri.andrea" <parri.andrea@gmail.com>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        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 Fri, Jan 13, 2023 at 05:05:11PM +0000, Jonas Oberhauser wrote:
> -----Original Message-----
> From: Alan Stern [mailto:stern@rowland.harvard.edu] 
> Sent: Friday, January 13, 2023 5:38 PM
> 
> > 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.
> 
> I see. I guess the other thing is more like a quiescent period.

"Quiescent period" was in fact my original name for "grace period"
back in the day, but a chorus of objections eventually prompted me to
instead label it a "grace period".

Perhaps you have given an improved rationale for their objections.  ;-)

>                                                                 I
> think the fact that RCU/safe memory reclamation(SMR) don't require a
> quiescent period is an important distinction, and even though we have
> our own SMR I never thought too deeply about this distinction.

If you want non-abysmal performance and scalability on modern hardware,
the distinction is critically important.  After all, the speed of light
really is finite, and atoms are of non-zero size.  And to the complete
surprise of my forty-years-ago self, these laws of physics seriously
constrain modern computing devices.

							Thanx, Paul