Re: [PATCH v2] parisc: Rewrite light-weight syscall and futex code

[Helge Deller <deller@xxxxxx>]

...
> In order to do this, we need a mechanism to trigger COW breaks outside the
> critical region. Fortunately, parisc has the "stbys,e" instruction. When
> the leftmost byte of a word is addressed, this instruction triggers all
> the exceptions of a normal store but it does not write to memory. Thus,
> we can use it to trigger COW breaks outside the critical region without
> modifying the data that is to be updated atomically.
...
> diff --git a/arch/parisc/include/asm/futex.h b/arch/parisc/include/asm/futex.h
> index 9cd4dd6e63ad..8f97db995b04 100644
> --- a/arch/parisc/include/asm/futex.h
> +++ b/arch/parisc/include/asm/futex.h
...
> +static inline bool _futex_force_interruptions(unsigned long ua)
> +{
> +	bool result;
> +
> +	__asm__ __volatile__(
> +		"1:\tldw 0(%1), %0\n"
> +		"2:\tstbys,e %%r0, 0(%1)\n"
> +		"\tcomclr,= %%r0, %%r0, %0\n"
> +		"3:\tldi 1, %0\n"
> +		ASM_EXCEPTIONTABLE_ENTRY(1b, 3b)
> +		ASM_EXCEPTIONTABLE_ENTRY(2b, 3b)
> +		: "=&r" (result) : "r" (ua) : "memory"
> +	);
> +	return result;

I wonder if we can get rid of the comclr,= instruction by using
ASM_EXCEPTIONTABLE_ENTRY_EFAULT instead of ASM_EXCEPTIONTABLE_ENTRY,
e.g.:

diff --git a/arch/parisc/include/asm/futex.h b/arch/parisc/include/asm/futex.h
index 8f97db995b04..ea052f013865 100644
--- a/arch/parisc/include/asm/futex.h
+++ b/arch/parisc/include/asm/futex.h
@@ -21,20 +21,21 @@ static inline unsigned long _futex_hash_index(unsigned long ua)
  * if load and store fault.
  */

-static inline bool _futex_force_interruptions(unsigned long ua)
+static inline unsigned long _futex_force_interruptions(unsigned long ua)
 {
-	bool result;
+	register unsigned long error __asm__ ("r8") = 0;
+	register unsigned long temp;

 	__asm__ __volatile__(
-		"1:\tldw 0(%1), %0\n"
-		"2:\tstbys,e %%r0, 0(%1)\n"
-		"\tcomclr,= %%r0, %%r0, %0\n"
-		"3:\tldi 1, %0\n"
-		ASM_EXCEPTIONTABLE_ENTRY(1b, 3b)
-		ASM_EXCEPTIONTABLE_ENTRY(2b, 3b)
-		: "=&r" (result) : "r" (ua) : "memory"
+		"1:\tldw 0(%2), %0\n"
+		"2:\tstbys,e %%r0, 0(%2)\n"
+		"3:\n"
+		ASM_EXCEPTIONTABLE_ENTRY_EFAULT(1b, 3b)
+		ASM_EXCEPTIONTABLE_ENTRY_EFAULT(2b, 3b)
+		: "=r" (temp), "=r" (error)
+		: "r" (ua), "1" (error) : "memory"
 	);
-	return result;
+	return error;
 }


Helge