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? > If you don't use ACCESS_ONCE(), the compiler really > is permitted to insert the "b = 999". So, why would the compiler do > such a thing? One possible reason would be from optimizations using > large registers to hold multiple values. A store from such a register > could clobber unrelated variables, but as long as the compiler fixes > up the clobbering after the fact, it is within its rights to do so. > > The sad fact is that the C standard really does permit the compiler > to assume that it is generating sequential code. Compiling the kernel requires quite a few extensions to the C standard. Assumptions about generating sequential code may well be among them. 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
