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Date: Wed, 20 Apr 2016 11:39:41 +0800
From: Pan Xinhui <xinhui@...ux.vnet.ibm.com>
To: Boqun Feng <boqun.feng@...il.com>
CC: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
tglx@...utronix.de
Subject: Re: [PATCH V2] powerpc: Implement {cmp}xchg for u8 and u16
Hello, boqun
On 2016年04月19日 17:18, Boqun Feng wrote:
> Hi Xinhui,
>
> On Tue, Apr 19, 2016 at 02:29:34PM +0800, Pan Xinhui wrote:
>> From: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
>>
>> Implement xchg{u8,u16}{local,relaxed}, and
>> cmpxchg{u8,u16}{,local,acquire,relaxed}.
>>
>> It works on all ppc.
>>
>
> Nice work!
>
thank you.
> 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@...radead.org>
>> Signed-off-by: Pan Xinhui <xinhui.pan@...ux.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; \
>> +}
a little optimization. Patch V3 will include your code, thanks.
>> +
>> /*
>> * 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.
>
I mean that. But I will think of the ordering issue for a while. :)
>> +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.
>
such cleanup work can be done in separated patch. Here I just make the compiler happy.
thanks
xinhui
> 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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