Re: [PATCH 1/24] make atomic_read() behave consistently on alpha
flat view
From: Paul E. McKenney <hidden>
Date: 2007-08-09 14:33:35
Also in:
linux-arch, lkml
On Thu, Aug 09, 2007 at 09:24:42AM -0400, Chris Snook wrote:
From: Chris Snook <redacted> Purify volatile use for atomic[64]_t on alpha.
Why not the same access-once semantics for atomic_set() as
for atomic_read()? As this patch stands, it might introduce
architecture-specific compiler-induced bugs due to the fact that
atomic_set() used to imply volatile behavior but no longer does.
See below for one approach to fixing this.
quoted hunk ↗ jump to hunk
Signed-off-by: Chris Snook <redacted>--- linux-2.6.23-rc2-orig/include/asm-alpha/atomic.h 2007-07-08 19:32:17.000000000 -0400 +++ linux-2.6.23-rc2/include/asm-alpha/atomic.h 2007-08-09 09:19:00.000000000 -0400@@ -14,18 +14,22 @@ /* - * Counter is volatile to make sure gcc doesn't try to be clever - * and move things around on us. We need to use _exactly_ the address - * the user gave us, not some alias that contains the same information. + * Make sure gcc doesn't try to be clever and move things around + * on us. We need to use _exactly_ the address the user gave us, + * not some alias that contains the same information. */ -typedef struct { volatile int counter; } atomic_t; -typedef struct { volatile long counter; } atomic64_t; +typedef struct { int counter; } atomic_t; +typedef struct { long counter; } atomic64_t; #define ATOMIC_INIT(i) ( (atomic_t) { (i) } ) #define ATOMIC64_INIT(i) ( (atomic64_t) { (i) } ) -#define atomic_read(v) ((v)->counter + 0) -#define atomic64_read(v) ((v)->counter + 0) +/* + * Casting to volatile here minimizes the need for barriers, + * without having to declare the type itself as volatile. + */ +#define atomic_read(v) (*(volatile int *)&(v)->counter + 0) +#define atomic64_read(v) (*(volatile long *)&(v)->counter + 0) #define atomic_set(v,i) ((v)->counter = (i)) #define atomic64_set(v,i) ((v)->counter = (i))
How about:
#define atomic_set(v,i) ((*(volatile int *)&(v)->counter) = (i))
#define atomic64_set(v,i) ((*(volatile long *)&(v)->counter) = (i))
Thanx, Paul
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