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Date:   Thu, 6 Jun 2019 14:14:38 +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 Wed, Jun 05, 2019 at 11:05:11PM -0700, Paul E. McKenney wrote:
> In case you were wondering, the reason that I was giving you such
> a hard time was that from what I could see, you were pushing for no
> {READ,WRITE}_ONCE() at all.  ;-)

Hmm, that's exactly what it should be in net/ipv4/inet_fragment.c.
We don't need the READ_ONCE/WRITE_ONCE (or volatile marking) at
all.  Even if the compiler dices and slices the reads/writes of
"a" into a thousand pieces, it should still work if the RCU
primitives are worth their salt.

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.

Email: Herbert Xu <>
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