On Mon, Mar 03, 2014 at 07:55:08PM +0100, Torvald Riegel wrote:
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> 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).
And I am OK with the value_dep_preserving type providing more/better
guarantees than we get by default from current compilers.
One question, though. Suppose that the code did not want a value
dependency to be tracked through a comparison operator. What does
the developer do in that case? (The reason I ask is that I have
not yet found a use case in the Linux kernel that expects a value
dependency to be tracked through a comparison.)
> 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.
Understood!
BTW, my current task is generating examples using the value_dep_preserving
type for RCU-protected array indexes.
Thanx, Paul
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