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Date:   Thu, 6 Jun 2019 17:28:55 +0800
From:   Herbert Xu <>
To:     "Paul E. McKenney" <>
Cc:     Alan Stern <>,
        Boqun Feng <>,
        Linus Torvalds <>,
        Frederic Weisbecker <>,
        Fengguang Wu <>, LKP <>,
        LKML <>,
        Netdev <>,
        "David S. Miller" <>,
        Andrea Parri <>,
        Luc Maranget <>,
        Jade Alglave <>
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.

Email: Herbert Xu <>
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