Subject: Re: linux-next: build warnings after merge of the access_once tree

From: Peter Zijlstra <[email-masked]>
Date: Thu, 26 Mar 2015 18:07:48 +0100

On Thu, Mar 26, 2015 at 09:45:07AM -0700, Linus Torvalds wrote:

> Stop this idiocy.

Yeah, clearly I can type faster than I can think straight :/

In any case, I've the below patch; do you want to take it now or do you
want me to route it through tip/locking/urgent or something like that?

---
Subject: kernel: Remove atomicy checks from {READ,WRITE}_ONCE
From: Peter Zijlstra <[email-masked]>
Date: Thu, 26 Mar 2015 17:45:37 +0100

The fact that volatile allows for atomic load/stores is a special case
not a requirement for {READ,WRITE}_ONCE(). Their primary purpose is to
force the compiler to emit load/stores _once_.

So remove the warning as it is correct behaviour. This also implies that
the u64 case is not 64bit only, so remove the #ifdef so we can generate
better code in that case.

Cc: Stephen Rothwell <[email-masked]>
Cc: Christian Borntraeger <[email-masked]>
Cc: Thomas Gleixner <[email-masked]>
Cc: Ingo Molnar <[email-masked]>
Cc: "H. Peter Anvin" <[email-masked]>
Cc: Davidlohr Bueso <[email-masked]>
Cc: Paul McKenney <[email-masked]>
Cc: Will Deacon <[email-masked]>
Acked-by: Linus Torvalds <[email-masked]>
Signed-off-by: Peter Zijlstra (Intel) <[email-masked]>
---
 include/linux/compiler.h | 16 ----------------
 1 file changed, 16 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 1b45e4a0519b..0e41ca0e5927 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -192,29 +192,16 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 
 #include <uapi/linux/types.h>
 
-static __always_inline void data_access_exceeds_word_size(void)
-#ifdef __compiletime_warning
-__compiletime_warning("data access exceeds word size and won't be atomic")
-#endif
-;
-
-static __always_inline void data_access_exceeds_word_size(void)
-{
-}
-
 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;
 	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
 	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
-#ifdef CONFIG_64BIT
 	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
-#endif
 	default:
 		barrier();
 		__builtin_memcpy((void *)res, (const void *)p, size);
-		data_access_exceeds_word_size();
 		barrier();
 	}
 }
@@ -225,13 +212,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
 	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
 	case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
-#ifdef CONFIG_64BIT
 	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
-#endif
 	default:
 		barrier();
 		__builtin_memcpy((void *)p, (const void *)res, size);
-		data_access_exceeds_word_size();
 		barrier();
 	}
 }