Re: [PATCH] Document Linux's memory barriers [try #5]

Linus Torvalds
2006-03-23T19:28:24Z

On Thu, 23 Mar 2006, David Howells wrote:
>
> > Some architectures have more expansive definition of data dependency,
> > including then- and else-clauses being data-dependent on the if-condition,
> > but this is probably too much detail.
>
> Linus calls that a "control dependency" and doesn't seem to think that's a
> problem as it's sorted out by branch prediction. What you said makes me
> wonder about conditional instructions (such as conditional move).

I'd put it the other way: a control dependency is not "sorted out" by
branch prediction, it is effectively _nullified_ by branch prediction.

Basically, control dependencies aren't dependencies at all. There is
absolutely _zero_ dependency between the following two loads:

if (load a)
load b;

because the "load b" can happen before the "load a" because of control
prediction.

So if you need a read barrier where there is a _control_ dependency in
between loading a and loading b, you need to make it a full "smp_rmb()".
It is _not_ sufficient to make this a "read_barrier_depends", because the
load of b really doesn't depend on the load of a at all.

So data dependencies that result in control dependencies aren't
dependencies at all. Not even if the address depends on the control
dependency.

So

int *address_of_b;

address_of_b = load(&a);
smp_read_barrier_depends();
b = load(address_of_b);

is correct, but

int *address_of_b = default_address_of_b;

if (load(&a))
address_of_b = another_address_of_b;
smp_read_barrier_depends();
b = load(address_of_b);

is NOT correct, because there is no data dependency on the load of b, just
a control dependency that the CPU may short-circuit with prediction, and
that second case thus needs a real smp_rmb().

And yes, if we ever hit a CPU that does actual data prediction, not just
control prediction, that will force smp_read_barrier_depends() to be the
same as smp_rmb() on such an architecture.

Linus