[tip:locking/core] locking, compiler.h: Cast away attributes in the WRITE_ONCE() magic

From: tip-bot for Christian Borntraeger
Date: Wed Aug 12 2015 - 08:34:53 EST

Commit-ID: ba33034fffc1189d95301bd865f1c799256e72a2
Gitweb: http://git.kernel.org/tip/ba33034fffc1189d95301bd865f1c799256e72a2
Author: Christian Borntraeger <borntraeger@xxxxxxxxxx>
AuthorDate: Tue, 4 Aug 2015 09:55:48 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 12 Aug 2015 11:58:58 +0200

locking, compiler.h: Cast away attributes in the WRITE_ONCE() magic

The kernel build bot showed a new warning triggered by commit:

76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in smp_store_release()/smp_load_acquire()")

because 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_ONCE().

(And as this now gets too long, also split the macro into multiple lines.)

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1438674948-38310-2-git-send-email-borntraeger@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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
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