On Fri, 2014-02-28 at 16:50 -0800, Paul E. McKenney wrote:
> +o Do not use the results from the boolean "&&" and "||" when
> + dereferencing. For example, the following (rather improbable)
> + code is buggy:
> +
> + int a[2];
> + int index;
> + int force_zero_index = 1;
> +
> + ...
> +
> + r1 = rcu_dereference(i1)
> + r2 = a[r1 && force_zero_index]; /* BUGGY!!! */
> +
> + The reason this is buggy is that "&&" and "||" are often compiled
> + using branches. While weak-memory machines such as ARM or PowerPC
> + do order stores after such branches, they can speculate loads,
> + which can result in misordering bugs.
> +
> +o Do not use the results from relational operators ("==", "!=",
> + ">", ">=", "<", or "<=") when dereferencing. For example,
> + the following (quite strange) code is buggy:
> +
> + int a[2];
> + int index;
> + int flip_index = 0;
> +
> + ...
> +
> + r1 = rcu_dereference(i1)
> + r2 = a[r1 != flip_index]; /* BUGGY!!! */
> +
> + As before, the reason this is buggy is that relational operators
> + are often compiled using branches. And as before, although
> + weak-memory machines such as ARM or PowerPC do order stores
> + after such branches, but can speculate loads, which can again
> + result in misordering bugs.
Those two would be allowed by the wording I have recently proposed,
AFAICS. r1 != flip_index would result in two possible values (unless
there are further constraints due to the type of r1 and the values that
flip_index can have).
I don't think the wording is flawed. We could raise the requirement of
having more than one value left for r1 to having more than N with N > 1
values left, but the fundamental problem remains in that a compiler
could try to generate a (big) switch statement.
Instead, I think that this indicates that the value_dep_preserving type
modifier would be useful: It would tell the compiler that it shouldn't
transform this into a branch in this case, yet allow that optimization
for all other code.
