linux-kernel - [PATCH 9/9] kernel: tighten rules for ACCESS ONCE

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Message-Id: <1417645821-54731-10-git-send-email-borntraeger@de.ibm.com>
Date:	Wed,  3 Dec 2014 23:30:21 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-arch@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCH 9/9] kernel: tighten rules for ACCESS ONCE

Now that all non-scalar users of ACCESS_ONCE have been converted
to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only
work on scalar types.
This variant was proposed by Alexei Starovoitov.

Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
---
 include/linux/compiler.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 947710e..5ba91de 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -441,8 +441,16 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
  * merging, or refetching absolutely anything at any time.  Its main intended
  * use is to mediate communication between process-level code and irq/NMI
  * handlers, all running on the same CPU.
+ *
+ * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
+ * on a union member will work as long as the size of the member matches the
+ * size of the union and the size is smaller than word size.
+ * If possible READ_ONCE/ASSIGN_ONCE should be used instead.
  */
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define __ACCESS_ONCE(x) ({ \
+	 __maybe_unused typeof(x) __var = 0; \
+	(volatile typeof(x) *)&(x); })
+#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
 
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES
-- 
1.9.3

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