Commit 112dc0c8 authored Aug 11, 2016 by Johannes Berg Committed by Ingo Molnar Aug 18, 2016

locking/barriers: Suppress sparse warnings in lockless_dereference()



After Peter's commit:

  331b6d8c ("locking/barriers: Validate lockless_dereference() is used on a pointer type")

... we get a lot of sparse warnings (one for every rcu_dereference, and more)
since the expression here is assigning to the wrong address space.

Instead of validating that 'p' is a pointer this way, instead make
it fail compilation when it's not by using sizeof(*(p)). This will
not cause any sparse warnings (tested, likely since the address
space is irrelevant for sizeof), and will fail compilation when
'p' isn't a pointer type.

Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 331b6d8c ("locking/barriers: Validate lockless_dereference() is used on a pointer type")
Link: http://lkml.kernel.org/r/1470909022-687-2-git-send-email-johannes@sipsolutions.net


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

parent f17b3ea3

include/linux/compiler.h

+3 −3

Original line number	Diff line number	Diff line
		@@ -527,13 +527,13 @@ 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
		* "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 *.
		* The seemingly unused size_t variable is to validate @p is indeed a pointer
		* type by making sure it can be dereferenced.
		*/
		#define lockless_dereference(p) \
		({ \
		typeof(p) _________p1 = READ_ONCE(p); \
		__maybe_unused const void * const _________p2 = _________p1; \
		size_t __maybe_unused __size_of_ptr = sizeof(*(p)); \
		smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
		(_________p1); \
		})

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