linux-kernel - [PATCHv2 09/10] tighten rules for ACCESS ONCE

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Date:	Tue, 25 Nov 2014 13:38:36 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	"linux-arch@...r.kernel.org, linux-mips@...ux-mips.org, linux-x86_64@...r.kernel.org, linux-s390"@vger.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	paulmck@...ux.vnet.ibm.com, mingo@...nel.org,
	torvalds@...ux-foundation.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	David Howells <dhowells@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCHv2 09/10] 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 0ff01f2..7265a6c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -411,8 +411,16 @@ static __always_inline void __read_once_size(volatile void *p, void *res, int si
  * 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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