Messages in this thread | | | Date | Thu, 6 Jun 2019 06:48:56 -0700 | From | "Paul E. McKenney" <> | Subject | Re: rcu_read_lock lost its compiler barrier |
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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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