On Wed, 6 Jun 2012, Paul E. McKenney wrote: > On Wed, Jun 06, 2012 at 11:44:50AM -0400, Alan Stern wrote: > > On Wed, 6 Jun 2012, Paul E. McKenney wrote: > > > > > > That just seems wrong. By the same reasoning, the compiler is within > > > > its rights to transform either the original code or the code using > > > > ACCESS_ONCE into: > > > > > > > > b = 999; > > > > if (a) > > > > b = 9; > > > > else > > > > b = 42; > > > > > > > > and again, other code would be confused. The simple fact is that > > > > SMP-safe code is not likely to be produced by a compiler that assumes > > > > everything is single-threaded. > > > > > > If you use ACCESS_ONCE(), the compiler is prohibited from inserting > > > the "b = 999". > > > > What prohibits it? > > The compiler cannot move a volatile access across a sequence point, for > example, across a statement boundary. How does inserting a store to a non-volatile value qualify as moving a volatile access? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
