linux-kernel - [tip:locking/urgent] locking/barriers: Suppress sparse warnings in lockless

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Date:	Thu, 18 Aug 2016 06:40:57 -0700
From:	tip-bot for Johannes Berg <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	johannes.berg@...el.com, akpm@...ux-foundation.org,
	tglx@...utronix.de, peterz@...radead.org, mingo@...nel.org,
	daniel.vetter@...el.com, hpa@...or.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	chris@...is-wilson.co.uk
Subject: [tip:locking/urgent] locking/barriers: Suppress sparse warnings in
 lockless_dereference()

Commit-ID:  112dc0c8069e5554e0ad29c58228f1e6ca49e13d
Gitweb:     http://git.kernel.org/tip/112dc0c8069e5554e0ad29c58228f1e6ca49e13d
Author:     Johannes Berg <johannes.berg@...el.com>
AuthorDate: Thu, 11 Aug 2016 11:50:22 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 18 Aug 2016 15:36:13 +0200

locking/barriers: Suppress sparse warnings in lockless_dereference()

After Peter's commit:

  331b6d8c7afc ("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@...ux.vnet.ibm.com>
Signed-off-by: Johannes Berg <johannes.berg@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Chris Wilson <chris@...is-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@...el.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: 331b6d8c7afc ("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@...nel.org>
---
 include/linux/compiler.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 1bb9548..436aa4e 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -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); \
 })