On Mon, Nov 02, 2015 at 10:08:24AM -0800, Linus Torvalds wrote: > On Mon, Nov 2, 2015 at 5:29 AM, Peter Zijlstra <[email protected]> wrote: > > +#define smp_cond_acquire(cond) do { \ > > + while (!(cond)) \ > > + cpu_relax(); \ > > + smp_read_barrier_depends(); /* ctrl */ \ > > + smp_rmb(); /* ctrl + rmb := acquire */ \ > > +} while (0) > > This code makes absolutely no sense. > > smp_read_barrier_depends() is about a memory barrier where there is a > data dependency between two accesses. The "depends" is very much about > the data dependency, and very much about *nothing* else.
Paul wasn't so sure, which I think is why smp_read_barrier_depends() is already used in, for example, READ_ONCE_CTRL: http://lkml.kernel.org/r/[email protected] although I agree that this would pave the way for speculative stores on Alpha and that seems like a heavy accusation to make. > Your comment talks about control dependencies, but > smp_read_barrier_depends() has absolutely nothing to do with a control > dependency. In fact, it is explicitly a no-op on architectures like > ARM and PowerPC that violate control dependencies. In this case, control dependencies are only referring to READ -> WRITE ordering, so they are honoured by ARM and PowerPC. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
