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Date: Thu, 6 Jun 2019 17:28:55 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
Boqun Feng <boqun.feng@...il.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>,
LKML <linux-kernel@...r.kernel.org>,
Netdev <netdev@...r.kernel.org>,
"David S. Miller" <davem@...emloft.net>,
Andrea Parri <andrea.parri@...rulasolutions.com>,
Luc Maranget <luc.maranget@...ia.fr>,
Jade Alglave <j.alglave@....ac.uk>
Subject: Re: rcu_read_lock lost its compiler barrier
On Thu, Jun 06, 2019 at 02:06:19AM -0700, Paul E. McKenney wrote:
>
> Or is your point instead that given the initial value of "a" being
> zero and the value stored to "a" being one, there is no way that
> any possible load and store tearing (your slicing and dicing) could
> possibly mess up the test of the value loaded from "a"?
Exactly. If you can dream up of a scenario where the compiler can
get this wrong I'm all ears.
> > But I do concede that in the general RCU case you must have the
> > READ_ONCE/WRITE_ONCE calls for rcu_dereference/rcu_assign_pointer.
>
> OK, good that we are in agreement on this part, at least! ;-)
Well only because we're allowing crazy compilers that can turn
a simple word-aligned word assignment (a = b) into two stores.
Cheers,
--
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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