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SubjectRe: [PATCH V2] powerpc: Implement {cmp}xchg for u8 and u16
Hi Xinhui,

On Tue, Apr 19, 2016 at 02:29:34PM +0800, Pan Xinhui wrote:
> From: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
>
> Implement xchg{u8,u16}{local,relaxed}, and
> cmpxchg{u8,u16}{,local,acquire,relaxed}.
>
> It works on all ppc.
>

Nice work!

AFAICT, your work doesn't depend on anything that ppc-specific, right?
So maybe we can use it as a general approach for a fallback
implementation on the archs without u8/u16 atomics. ;-)

> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
> ---
> change from V1:
> rework totally.
> ---
> arch/powerpc/include/asm/cmpxchg.h | 83 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 83 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
> index 44efe73..79a1f45 100644
> --- a/arch/powerpc/include/asm/cmpxchg.h
> +++ b/arch/powerpc/include/asm/cmpxchg.h
> @@ -7,6 +7,37 @@
> #include <asm/asm-compat.h>
> #include <linux/bug.h>
>
> +#ifdef __BIG_ENDIAN
> +#define BITOFF_CAL(size, off) ((sizeof(u32) - size - off) * BITS_PER_BYTE)
> +#else
> +#define BITOFF_CAL(size, off) (off * BITS_PER_BYTE)
> +#endif
> +
> +static __always_inline unsigned long
> +__cmpxchg_u32_local(volatile unsigned int *p, unsigned long old,
> + unsigned long new);
> +
> +#define __XCHG_GEN(cmp, type, sfx, u32sfx, skip, v) \
> +static __always_inline u32 \
> +__##cmp##xchg_##type##sfx(v void *ptr, u32 old, u32 new) \
> +{ \
> + int size = sizeof (type); \
> + int off = (unsigned long)ptr % sizeof(u32); \
> + volatile u32 *p = ptr - off; \
> + int bitoff = BITOFF_CAL(size, off); \
> + u32 bitmask = ((0x1 << size * BITS_PER_BYTE) - 1) << bitoff; \
> + u32 oldv, newv; \
> + u32 ret; \
> + do { \
> + oldv = READ_ONCE(*p); \
> + ret = (oldv & bitmask) >> bitoff; \
> + if (skip && ret != old) \
> + break; \
> + newv = (oldv & ~bitmask) | (new << bitoff); \
> + } while (__cmpxchg_u32##u32sfx((v void*)p, oldv, newv) != oldv);\

Forgive me if this is too paranoid, but I think we can save the
READ_ONCE() in the loop if we change the code into the following,
because cmpxchg will return the "new" value, if the cmp part fails.

newv = READ_ONCE(*p);

do {
oldv = newv;
ret = (oldv & bitmask) >> bitoff;
if (skip && ret != old)
break;
newv = (oldv & ~bitmask) | (new << bitoff);
newv = __cmpxchg_u32##u32sfx((void *)p, oldv, newv);
} while(newv != oldv);

> + return ret; \
> +}
> +
> /*
> * Atomic exchange
> *
> @@ -14,6 +45,19 @@
> * the previous value stored there.
> */
>
> +#define XCHG_GEN(type, sfx, v) \
> + __XCHG_GEN(_, type, sfx, _local, 0, v) \
^^^^^^^

This should be sfx, right? Otherwise, all the newly added xchg will
call __cmpxchg_u32_local, this will result in wrong ordering guarantees.

> +static __always_inline u32 __xchg_##type##sfx(v void *p, u32 n) \
> +{ \
> + return ___xchg_##type##sfx(p, 0, n); \
> +}
> +
> +XCHG_GEN(u8, _local, volatile);

I don't think we need the "volatile" modifier here, because READ_ONCE()
and __cmpxchg_u32_* all have "volatile" semantics IIUC, so maybe we can
save a paramter for the __XCHG_GEN macro.

Regards,
Boqun

> +XCHG_GEN(u8, _relaxed, );
> +XCHG_GEN(u16, _local, volatile);
> +XCHG_GEN(u16, _relaxed, );
> +#undef XCHG_GEN
> +
> static __always_inline unsigned long
> __xchg_u32_local(volatile void *p, unsigned long val)
> {
> @@ -88,6 +132,10 @@ static __always_inline unsigned long
> __xchg_local(volatile void *ptr, unsigned long x, unsigned int size)
> {
> switch (size) {
> + case 1:
> + return __xchg_u8_local(ptr, x);
> + case 2:
> + return __xchg_u16_local(ptr, x);
> case 4:
> return __xchg_u32_local(ptr, x);
> #ifdef CONFIG_PPC64
> @@ -103,6 +151,10 @@ static __always_inline unsigned long
> __xchg_relaxed(void *ptr, unsigned long x, unsigned int size)
> {
> switch (size) {
> + case 1:
> + return __xchg_u8_relaxed(ptr, x);
> + case 2:
> + return __xchg_u16_relaxed(ptr, x);
> case 4:
> return __xchg_u32_relaxed(ptr, x);
> #ifdef CONFIG_PPC64
> @@ -226,6 +278,21 @@ __cmpxchg_u32_acquire(u32 *p, unsigned long old, unsigned long new)
> return prev;
> }
>
> +
> +#define CMPXCHG_GEN(type, sfx, v) \
> + __XCHG_GEN(cmp, type, sfx, sfx, 1, v)
> +
> +CMPXCHG_GEN(u8, , volatile);
> +CMPXCHG_GEN(u8, _local, volatile);
> +CMPXCHG_GEN(u8, _relaxed, );
> +CMPXCHG_GEN(u8, _acquire, );
> +CMPXCHG_GEN(u16, , volatile);
> +CMPXCHG_GEN(u16, _local, volatile);
> +CMPXCHG_GEN(u16, _relaxed, );
> +CMPXCHG_GEN(u16, _acquire, );
> +#undef CMPXCHG_GEN
> +#undef __XCHG_GEN
> +
> #ifdef CONFIG_PPC64
> static __always_inline unsigned long
> __cmpxchg_u64(volatile unsigned long *p, unsigned long old, unsigned long new)
> @@ -316,6 +383,10 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,
> unsigned int size)
> {
> switch (size) {
> + case 1:
> + return __cmpxchg_u8(ptr, old, new);
> + case 2:
> + return __cmpxchg_u16(ptr, old, new);
> case 4:
> return __cmpxchg_u32(ptr, old, new);
> #ifdef CONFIG_PPC64
> @@ -332,6 +403,10 @@ __cmpxchg_local(volatile void *ptr, unsigned long old, unsigned long new,
> unsigned int size)
> {
> switch (size) {
> + case 1:
> + return __cmpxchg_u8_local(ptr, old, new);
> + case 2:
> + return __cmpxchg_u16_local(ptr, old, new);
> case 4:
> return __cmpxchg_u32_local(ptr, old, new);
> #ifdef CONFIG_PPC64
> @@ -348,6 +423,10 @@ __cmpxchg_relaxed(void *ptr, unsigned long old, unsigned long new,
> unsigned int size)
> {
> switch (size) {
> + case 1:
> + return __cmpxchg_u8_relaxed(ptr, old, new);
> + case 2:
> + return __cmpxchg_u16_relaxed(ptr, old, new);
> case 4:
> return __cmpxchg_u32_relaxed(ptr, old, new);
> #ifdef CONFIG_PPC64
> @@ -364,6 +443,10 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
> unsigned int size)
> {
> switch (size) {
> + case 1:
> + return __cmpxchg_u8_acquire(ptr, old, new);
> + case 2:
> + return __cmpxchg_u16_acquire(ptr, old, new);
> case 4:
> return __cmpxchg_u32_acquire(ptr, old, new);
> #ifdef CONFIG_PPC64
> --
> 2.4.3
>

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