Messages in this thread | | | Date | Thu, 23 Mar 2006 11:28:04 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH] Document Linux's memory barriers [try #5] |
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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.
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