[llvmlinux] Fail to build linux-next

Vinícius Tinti viniciustinti at gmail.com
Mon Mar 9 02:28:32 UTC 2015

This is the patch responsible: dd36929720f40f17685e841ae0d4c581c165ea60

commit dd36929720f40f17685e841ae0d4c581c165ea60
Author: Linus Torvalds <torvalds at linux-foundation.org>
Date:   Fri Feb 20 15:46:31 2015 -0800

    kernel: make READ_ONCE() valid on const arguments

    The use of READ_ONCE() causes lots of warnings witht he pending paravirt
    spinlock fixes, because those ends up having passing a member to a
    'const' structure to READ_ONCE().

    There should certainly be nothing wrong with using READ_ONCE() with a
    const source, but the helper function __read_once_size() would cause
    warnings because it would drop the 'const' qualifier, but also because
    the destination would be marked 'const' too due to the use of 'typeof'.

    Use a union of types in READ_ONCE() to avoid this issue.

    Also make sure to use parenthesis around the macro arguments to avoid
    possible operator precedence issues.

    Tested-by: Ingo Molnar <mingo at kernel.org>
    Cc: Christian Borntraeger <borntraeger at de.ibm.com>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index d1ec10a..1b45e4a 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -202,7 +202,7 @@ static __always_inline void

-static __always_inline void __read_once_size(volatile void *p, void *res,
int size)
+static __always_inline void __read_once_size(const volatile void *p, void
*res, int size)
        switch (size) {
        case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
@@ -259,10 +259,10 @@ static __always_inline void
__write_once_size(volatile void *p, void *res, int s

 #define READ_ONCE(x) \
-       ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val));
__val; })
+       ({ union { typeof(x) __val; char __c[1]; } __u;
__read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })

 #define WRITE_ONCE(x, val) \
-       ({ typeof(x) __val; __val = val; __write_once_size(&x, &__val,
sizeof(__val)); __val; })
+       ({ typeof(x) __val = (val); __write_once_size(&(x), &__val,
sizeof(__val)); __val; })

 #endif /* __KERNEL__ */

Simplicity is the ultimate sophistication
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