Messages in this thread | | | Date | Thu, 9 Aug 2007 07:32:55 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/24] make atomic_read() behave consistently on alpha |
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On Thu, Aug 09, 2007 at 09:24:42AM -0400, Chris Snook wrote: > From: Chris Snook <csnook@redhat.com> > > 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.
> Signed-off-by: Chris Snook <csnook@redhat.com> > > --- 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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