locking: Remove atomicy checks from {READ,WRITE}_ONCE

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_.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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();
 	}
 }