Commit d7127b5e authored Aug 26, 2016 by Johannes Berg Committed by Ingo Molnar Sep 05, 2016

locking/barriers: Don't use sizeof(void) in lockless_dereference()



My previous commit:

  112dc0c8 ("locking/barriers: Suppress sparse warnings in lockless_dereference()")

caused sparse to complain that (in radix-tree.h) we use sizeof(void)
since that rcu_dereference()s a void *.

Really, all we need is to have the expression *p in here somewhere
to make sure p is a pointer type, and sizeof(*p) was the thing that
came to my mind first to make sure that's done without really doing
anything at runtime.

Another thing I had considered was using typeof(*p), but obviously
we can't just declare a typeof(*p) variable either, since that may
end up being void. Declaring a variable as typeof(*p)* gets around
that, and still checks that typeof(*p) is valid, so do that. This
type construction can't be done for _________p1 because that will
actually be used and causes sparse address space warnings, so keep
a separate unused variable for it.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kbuild-all@01.org
Fixes: 112dc0c8 ("locking/barriers: Suppress sparse warnings in lockless_dereference()")
Link: http://lkml.kernel.org/r/1472192160-4049-1-git-send-email-johannes@sipsolutions.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent c6935931

include/linux/compiler.h

+4 −3

Original line number	Original line	Diff line number	Diff line
	@@ -527,13 +527,14 @@ static __always_inline void __write_once_size(volatile void p, void res, int s
	* object's lifetime is managed by something other than RCU. That		* object's lifetime is managed by something other than RCU. That
	* "something other" might be reference counting or simple immortality.		* "something other" might be reference counting or simple immortality.
	*		*
	* The seemingly unused size_t variable is to validate @p is indeed a pointer		* The seemingly unused variable ___typecheck_p validates that @p is
	* type by making sure it can be dereferenced.		* indeed a pointer type by using a pointer to typeof(*p) as the type.
			* Taking a pointer to typeof(p) again is needed in case p is void .
	*/		*/
	#define lockless_dereference(p) \		#define lockless_dereference(p) \
	({ \		({ \
	typeof(p) _________p1 = READ_ONCE(p); \		typeof(p) _________p1 = READ_ONCE(p); \
	size_t __maybe_unused __size_of_ptr = sizeof(*(p)); \		typeof((p)) ___typecheck_p __maybe_unused; \
	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \		smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
	(_________p1); \		(_________p1); \
	})		})

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