netdev - Re: rcu_read_lock lost its compiler barrier

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Message-ID: <20190606133824.aibysezb5qdo3x27@gondor.apana.org.au>
Date:   Thu, 6 Jun 2019 21:38:24 +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 03:58:17AM -0700, Paul E. McKenney wrote:
>
> I cannot immediately think of a way that the compiler could get this
> wrong even in theory, but similar code sequences can be messed up.
> The reason for this is that in theory, the compiler could use the
> stored-to location as temporary storage, like this:
> 
> 	a = whatever;	// Compiler uses "a" as a temporary
> 	do_something();
> 	whatever = a;
> 	a = 1;		// Intended store

Well if the compiler is going to do this then surely it would
continue to do this even if you used WRITE_ONCE.  Remember a is
not volatile, only the access of a through WRITE_ONCE is volatile.

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