On Fri, 18 Jan 2013 23:31:43 +0100
Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
> From: Johannes Berg <johannes.berg@xxxxxxxxx>
>
> I recently made the mistake of writing:
>
> foo = lockdep_dereference_protected(..., lockdep_assert_held(...));
>
> which is clearly bogus. If lockdep is disabled in the
> config this would cause a compile failure, if it is
> enabled then it compiles and causes a puzzling warning
> about dereferencing without the correct protection.
>
> Wrap the macro in "do { ... } while (0)" to also fail
> compile for this when lockdep is enabled.
>
> ...
>
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -359,7 +359,9 @@ extern void lockdep_trace_alloc(gfp_t mask);
>
> #define lockdep_depth(tsk) (debug_locks ? (tsk)->lockdep_depth : 0)
>
> -#define lockdep_assert_held(l) WARN_ON(debug_locks && !lockdep_is_held(l))
> +#define lockdep_assert_held(l) do { \
> + WARN_ON(debug_locks && !lockdep_is_held(l)); \
> + } while (0)
>
> #define lockdep_recursing(tsk) ((tsk)->lockdep_recursion)
A sane fix would be to convert lockdep_assert_held() into a static
inline void C function. But, alas, much of the lockdep API is designed
to work on "any type which has a field called dep_map", which was
rather a silly stunt IMO.
lockdep_depth() and lockdep_assert_held() may or may not evaluate their
argument, and this is runtime controllable. whee.
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