Re: [PATCH] locking/barriers: evaluate the argument to lockless_dereference only once

[Peter Zijlstra <peterz@infradead.org>]

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 <peterz@infradead.org>
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) <peterz@infradead.org>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/20160522104827.GP3193@twins.programming.kicks-ass.net
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

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); \
 })