linux-kernel - [PATCH] compiler.h: cast away attributes in WRITE

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Date:	Tue,  4 Aug 2015 09:55:48 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrey Konovalov <andreyknvl@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCH] compiler.h: cast away attributes in WRITE_ONCE magic

kernel build bot showed a warning triggered by commit
76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in
smp_store_release()/smp_load_acquire()"). Turns out that sparse
does not like WRITE_ONCE accessing elements from the (sparse)
rcu address space.

fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
fs/afs/inode.c:448:9:    expected struct afs_permits *__val
fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>

Solution is to force cast away the sparse attributes for the initializer
of the union in WRITE_SAME. As this now gets too long, lets split
the macro.

Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
---
 include/linux/compiler.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e08a6ae..c836eb2 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #define WRITE_ONCE(x, val) \
-	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+({							\
+	union { typeof(x) __val; char __c[1]; } __u =	\
+		{ .__val = (__force typeof(x)) (val) }; \
+	__write_once_size(&(x), __u.__c, sizeof(x));	\
+	__u.__val;					\
+})
 
 /**
  * READ_ONCE_CTRL - Read a value heading a control dependency
-- 
2.3.8

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