Re: rcu_read_lock lost its compiler barrier

From: Herbert Xu
Date: Thu Jun 06 2019 - 09:42:48 EST

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.

Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page:
PGP Key: