[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <Y9GSuzHFdRfLeAAp@andrea>
Date: Wed, 25 Jan 2023 21:36:11 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Alan Stern <stern@...land.harvard.edu>,
Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
Jonas Oberhauser <jonas.oberhauser@...wei.com>,
Peter Zijlstra <peterz@...radead.org>, 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)
> > Why do you want the implementation to forbid it? The pattern of the
> > litmus test resembles 3+3W, and you don't care whether the kernel allows
> > that pattern. Do you?
>
> Jonas asked a similar question, so I am answering you both here.
>
> With (say) a release-WRITE_ONCE() chain implementing N+2W for some
> N, it is reasonably well known that you don't get ordering, hardware
> support otwithstanding. After all, none of the Linux kernel, C, and C++
> memory models make that guarantee. In addition, the non-RCU barriers
> and accesses that you can use to create N+2W have been in very wide use
> for a very long time.
>
> Although RCU has been in use for almost as long as those non-RCU barriers,
> it has not been in wide use for anywhere near that long. So I cannot
> be so confident in ruling out some N+2W use case for RCU.
Did some archeology... the pattern, with either RCU sync plus a release
or with two full fences plus a release, was forbidden by "ancient LKMM":
the relevant changes were described in
https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/WeakModel.html#Coherence%20Point%20and%20RCU
Andrea
Powered by blists - more mailing lists