On Tue, Jun 07, 2016 at 03:14:55PM +0200, Mateusz Guzik wrote:
> The commit 25841ee0e9d2a ("Validate lockless_dereference() is used on a
> pointer type") added a second use of the parameter to the macro.
>
> This leads to trouble with consumers which use arguments with side
> effects.
Current tip/locking/core doesn't have that patch anymore IIRC, Ingo
folded your thing and we now have:
commit 331b6d8c7afc2e5b900b9dcd850c265e1ba8d8e7
Author: Peter Zijlstra
Date: Sun May 22 12:48:27 2016 +0200
locking/barriers: Validate lockless_dereference() is used on a pointer type
Use the type to validate the argument @p is indeed a pointer type.
Signed-off-by: Peter Zijlstra (Intel)
Cc: Alexey Dobriyan
Cc: Andrew Morton
Cc: Linus Torvalds
Cc: Paul E. McKenney
Cc: Paul McKenney
Cc: Peter Zijlstra
Cc: Thomas Gleixner
Link:
http://lkml.kernel.org/r/20160522104827.gp3...@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 793c0829e3a3..06f27fd9d760 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -545,10 +545,14 @@ static __always_inline void __write_once_size(volatile
void *p, void *res, int s
* Similar to rcu_dereference(), but for situations where the pointed-to
* object's lifetime is managed by something other than RCU. That
* "something other" might be reference counting or simple immortality.
+ *
+ * The seemingly unused void * variable is to validate @p is indeed a pointer
+ * type. All pointer types silently cast to void *.
*/
#define lockless_dereference(p) \
({ \
typeof(p) _p1 = READ_ONCE(p); \
+ __maybe_unused const void * const _p2 = _p1; \
smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
(_p1); \
})
