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

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

  76695af ("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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: borntraeger

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