Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()

[Heiko Carstens]

On Mon, Dec 19, 2022 at 04:35:30PM +0100, Peter Zijlstra wrote:
> For all architectures that currently support cmpxchg_double()
> implement the cmpxchg128() family of functions that is basically the
> same but with a saner interface.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/atomic_ll_sc.h |   38 +++++++++++++++++++++++
>  arch/arm64/include/asm/atomic_lse.h   |   33 +++++++++++++++++++-
>  arch/arm64/include/asm/cmpxchg.h      |   26 ++++++++++++++++
>  arch/s390/include/asm/cmpxchg.h       |   33 ++++++++++++++++++++
>  arch/x86/include/asm/cmpxchg_32.h     |    3 +
>  arch/x86/include/asm/cmpxchg_64.h     |   55 +++++++++++++++++++++++++++++++++-
>  6 files changed, 185 insertions(+), 3 deletions(-)
...
> --- a/arch/s390/include/asm/cmpxchg.h
> +++ b/arch/s390/include/asm/cmpxchg.h
> @@ -201,4 +201,37 @@ static __always_inline int __cmpxchg_dou
>  			 (unsigned long)(n1), (unsigned long)(n2));	\
>  })
>  
> +#define system_has_cmpxchg128()		1
> +
> +static __always_inline u128 arch_cmpxchg128(volatile u128 *ptr, u128 old, u128 new)
> +{
> +	asm volatile(
> +		"	cdsg	%[old],%[new],%[ptr]\n"
> +		: [old] "+&d" (old)
> +		: [new] "d" (new),
> +		  [ptr] "QS" (*(unsigned long *)ptr)
> +		: "memory", "cc");
> +	return old;
> +}
> +
> +static __always_inline bool arch_try_cmpxchg128(volatile u128 *ptr, u128 *oldp, u128 new)
> +{
> +	u128 old = *oldp;
> +	int cc;
> +
> +	asm volatile(
> +		"	cdsg	%[old],%[new],%[ptr]\n"
> +		"	ipm	%[cc]\n"
> +		"	srl	%[cc],28\n"
> +		: [cc] "=&d" (cc), [old] "+&d" (old)
> +		: [new] "d" (new),
> +		  [ptr] "QS" (*(unsigned long *)ptr)
> +		: "memory", "cc");
> +
> +	if (unlikely(!cc))
> +		*oldp = old;
> +
> +	return likely(cc);
> +}
> +

I was wondering why arch_try_cmpxchg128() isn't even used with later
code. Turned out this is because of a missing

#define arch_try_cmpxchg128 arch_try_cmpxchg128

which in turn means that the generic fallback variant is used.

The above arch_try_cmpxchg128() implementation is broken, since it has
inversed condition code handling (cc == 0 means compare and swap
succeeded, cc == 1 means it failed).

However I would prefer to use the generic fallback variant anyway.
Could you please merge the below into your current patch?

It addresses also the oddity that *ptr within arch_cmpxchg128() is
only specified as input, while it should be input/output - it doesn't
matter due to the memory clobber, but let's have that correct anyway.

diff --git a/arch/s390/include/asm/cmpxchg.h b/arch/s390/include/asm/cmpxchg.h
index 527c968945e8..0b98f57bbe9e 100644
--- a/arch/s390/include/asm/cmpxchg.h
+++ b/arch/s390/include/asm/cmpxchg.h
@@ -173,31 +173,12 @@ static __always_inline u128 arch_cmpxchg128(volatile u128 *ptr, u128 old, u128 n
 {
 	asm volatile(
 		"	cdsg	%[old],%[new],%[ptr]\n"
-		: [old] "+&d" (old)
-		: [new] "d" (new),
-		  [ptr] "QS" (*(u128 *)ptr)
+		: [old] "+d" (old), [ptr] "+QS" (*ptr)
+		: [new] "d" (new)
 		: "memory", "cc");
 	return old;
 }
 
-static __always_inline bool arch_try_cmpxchg128(volatile u128 *ptr, u128 *oldp, u128 new)
-{
-	u128 old = *oldp;
-	int cc;
-
-	asm volatile(
-		"	cdsg	%[old],%[new],%[ptr]\n"
-		"	ipm	%[cc]\n"
-		"	srl	%[cc],28\n"
-		: [cc] "=&d" (cc), [old] "+&d" (old)
-		: [new] "d" (new),
-		  [ptr] "QS" (*(u128 *)ptr)
-		: "memory", "cc");
-
-	if (unlikely(!cc))
-		*oldp = old;
-
-	return likely(cc);
-}
+#define arch_cmpxchg128		arch_cmpxchg128
 
 #endif /* __ASM_CMPXCHG_H */