On Sat, Jun 14, 2008 at 11:36 AM, Helge Deller <deller@xxxxxx> wrote:
> I'm wondering if this patch might help people who are seeing locking
> problems on SMP boxes ?
> Helge
>
> diff --git a/include/asm-parisc/system.h b/include/asm-parisc/system.h
> index ee80c92..4752684 100644
> --- a/include/asm-parisc/system.h
> +++ b/include/asm-parisc/system.h
> @@ -168,8 +168,9 @@ static inline void set_eiem(unsigned long val)
> /* LDCW, the only atomic read-write operation PA-RISC has. *sigh*. */
> #define __ldcw(a) ({ \
> unsigned __ret; \
> - __asm__ __volatile__(__LDCW " 0(%1),%0" \
> - : "=r" (__ret) : "r" (a)); \
> + __asm__ __volatile__(__LDCW " 0(%2),%0" \
> + : "=r" (__ret), "=m" (*(a)) \
> + : "r" (a), "m" (*(a)) ); \
> __ret; \
> })
You don't want to do that, the compiler might hold "=m" (*(a)) in a
temporary memory location.
e.g.
temp_mem = *a;
reg2 = &temp_mem;
... operation ...
*a = temp_mem;
You would be atomic with regards to the store to temp_mem, but not a.
Infact this could always be the case.
I'm now of the opinion that we need a "memory" clobber in the original
expression to prevent this from ever happening.
Cheers,
Carlos.
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