linux-kernel - Re: [tip:locking/core] locking/pvqspinlock: Replace xchg() by the more descriptive set

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Message-ID: <20150512145939.GG16478@twins.programming.kicks-ass.net>
Date:	Tue, 12 May 2015 16:59:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	doug.hatch@...com, tglx@...utronix.de, mingo@...nel.org,
	Waiman.Long@...com, linux-kernel@...r.kernel.org,
	scott.norton@...com, torvalds@...ux-foundation.org, hpa@...or.com
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/core] locking/pvqspinlock: Replace xchg() by the
 more descriptive set_mb()

On Mon, May 11, 2015 at 04:54:08PM +0200, Peter Zijlstra wrote:

> +++ b/arch/arm/include/asm/barrier.h
> @@ -81,7 +81,7 @@ do {									\
>  #define read_barrier_depends()		do { } while(0)
>  #define smp_read_barrier_depends()	do { } while(0)
>  
> -#define set_mb(var, value)	do { var = value; smp_mb(); } while (0)
> +#define set_mb(var, value)	do { WRITE_ONCE(var, value); smp_mb(); } while (0)
>  
>  #define smp_mb__before_atomic()	smp_mb()
>  #define smp_mb__after_atomic()	smp_mb()

Which triggered a massive compile fail and requires the same union
trickery we already had for READ_ONCE().

Added the below bit to the patch to aid compiling.

---
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index a7c0941d10da..03e227ba481c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -250,7 +250,7 @@ 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) \
-	({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
+	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #endif /* __KERNEL__ */
 
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