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Date: Thu, 6 Jun 2019 06:48:56 -0700 From: "Paul E. McKenney" <paulmck@...ux.ibm.com> To: Herbert Xu <herbert@...dor.apana.org.au> 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 09:38:24PM +0800, Herbert Xu wrote: > 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. I disagree. Given a volatile store, the compiler cannot assume that the stored-to location is normal memory at that point in time, and therefore cannot assume that it is safe to invent a store to that location (as shown above). Thus far, the C++ standards committee seems on-board with this, though time will tell. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1382r1.pdf Thanx, Paul
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