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Subject[RFC PATCH] locking/atomics/powerpc: Introduce optimized cmpxchg_release() family of APIs for PowerPC

* Ingo Molnar <mingo@kernel.org> wrote:

> > So there's no loss in arch flexibility.
>
> BTW., PowerPC for example is already in such a situation, it does not define
> atomic_cmpxchg_release(), only the other APIs:
>
> #define atomic_cmpxchg(v, o, n) (cmpxchg(&((v)->counter), (o), (n)))
> #define atomic_cmpxchg_relaxed(v, o, n) \
> cmpxchg_relaxed(&((v)->counter), (o), (n))
> #define atomic_cmpxchg_acquire(v, o, n) \
> cmpxchg_acquire(&((v)->counter), (o), (n))
>
> Was it really the intention on the PowerPC side that the generic code falls back
> to cmpxchg(), i.e.:
>
> # define atomic_cmpxchg_release(...) __atomic_op_release(atomic_cmpxchg, __VA_ARGS__)
>
> Which after macro expansion becomes:
>
> smp_mb__before_atomic();
> atomic_cmpxchg_relaxed(v, o, n);
>
> smp_mb__before_atomic() on PowerPC falls back to the generic __smp_mb(), which
> falls back to mb(), which on PowerPC is the 'sync' instruction.
>
> Isn't this a inefficiency bug?
>
> While I'm pretty clueless about PowerPC low level cmpxchg atomics, they appear to
> have the following basic structure:
>
> full cmpxchg():
>
> PPC_ATOMIC_ENTRY_BARRIER # sync
> ldarx + stdcx
> PPC_ATOMIC_EXIT_BARRIER # sync
>
> cmpxchg_relaxed():
>
> ldarx + stdcx
>
> cmpxchg_acquire():
>
> ldarx + stdcx
> PPC_ACQUIRE_BARRIER # lwsync
>
> The logical extension for cmpxchg_release() would be:
>
> cmpxchg_release():
>
> PPC_RELEASE_BARRIER # lwsync
> ldarx + stdcx
>
> But instead we silently get the generic fallback, which does:
>
> smp_mb__before_atomic();
> atomic_cmpxchg_relaxed(v, o, n);
>
> Which maps to:
>
> sync
> ldarx + stdcx
>
> Note that it uses a full barrier instead of lwsync (does that stand for
> 'lightweight sync'?).
>
> Even if it turns out we need the full barrier, with the overly finegrained
> structure of the atomics this detail is totally undocumented and non-obvious.

The patch below fills in those bits and implements the optimized cmpxchg_release()
family of APIs. The end effect should be that cmpxchg_release() will now use
'lwsync' instead of 'sync' on PowerPC, for the following APIs:

cmpxchg_release()
cmpxchg64_release()
atomic_cmpxchg_release()
atomic64_cmpxchg_release()

I based this choice of the release barrier on an existing bitops low level PowerPC
method:

DEFINE_BITOP(clear_bits_unlock, andc, PPC_RELEASE_BARRIER)

This clearly suggests that PPC_RELEASE_BARRIER is in active use and 'lwsync' is
the 'release barrier' instruction, if I interpreted that right.

But I know very little about PowerPC so this might be spectacularly wrong. It's
totally untested as well. I also pretty sick today so my mental capabilities are
significantly reduced ...

So not signed off and such.

Thanks,

Ingo

---
arch/powerpc/include/asm/atomic.h | 4 ++
arch/powerpc/include/asm/cmpxchg.h | 81 ++++++++++++++++++++++++++++++++++++++
2 files changed, 85 insertions(+)

diff --git a/arch/powerpc/include/asm/atomic.h b/arch/powerpc/include/asm/atomic.h
index 682b3e6a1e21..f7a6f29acb12 100644
--- a/arch/powerpc/include/asm/atomic.h
+++ b/arch/powerpc/include/asm/atomic.h
@@ -213,6 +213,8 @@ static __inline__ int atomic_dec_return_relaxed(atomic_t *v)
cmpxchg_relaxed(&((v)->counter), (o), (n))
#define atomic_cmpxchg_acquire(v, o, n) \
cmpxchg_acquire(&((v)->counter), (o), (n))
+#define atomic_cmpxchg_release(v, o, n) \
+ cmpxchg_release(&((v)->counter), (o), (n))

#define atomic_xchg(v, new) (xchg(&((v)->counter), new))
#define atomic_xchg_relaxed(v, new) xchg_relaxed(&((v)->counter), (new))
@@ -519,6 +521,8 @@ static __inline__ long atomic64_dec_if_positive(atomic64_t *v)
cmpxchg_relaxed(&((v)->counter), (o), (n))
#define atomic64_cmpxchg_acquire(v, o, n) \
cmpxchg_acquire(&((v)->counter), (o), (n))
+#define atomic64_cmpxchg_release(v, o, n) \
+ cmpxchg_release(&((v)->counter), (o), (n))

#define atomic64_xchg(v, new) (xchg(&((v)->counter), new))
#define atomic64_xchg_relaxed(v, new) xchg_relaxed(&((v)->counter), (new))
diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
index 9b001f1f6b32..6e46310b1833 100644
--- a/arch/powerpc/include/asm/cmpxchg.h
+++ b/arch/powerpc/include/asm/cmpxchg.h
@@ -213,10 +213,12 @@ __xchg_relaxed(void *ptr, unsigned long x, unsigned int size)
CMPXCHG_GEN(u8, , PPC_ATOMIC_ENTRY_BARRIER, PPC_ATOMIC_EXIT_BARRIER, "memory");
CMPXCHG_GEN(u8, _local, , , "memory");
CMPXCHG_GEN(u8, _acquire, , PPC_ACQUIRE_BARRIER, "memory");
+CMPXCHG_GEN(u8, _release, PPC_RELEASE_BARRIER, , "memory");
CMPXCHG_GEN(u8, _relaxed, , , "cc");
CMPXCHG_GEN(u16, , PPC_ATOMIC_ENTRY_BARRIER, PPC_ATOMIC_EXIT_BARRIER, "memory");
CMPXCHG_GEN(u16, _local, , , "memory");
CMPXCHG_GEN(u16, _acquire, , PPC_ACQUIRE_BARRIER, "memory");
+CMPXCHG_GEN(u16, _release, PPC_RELEASE_BARRIER, , "memory");
CMPXCHG_GEN(u16, _relaxed, , , "cc");

static __always_inline unsigned long
@@ -314,6 +316,29 @@ __cmpxchg_u32_acquire(u32 *p, unsigned long old, unsigned long new)
return prev;
}

+static __always_inline unsigned long
+__cmpxchg_u32_release(u32 *p, unsigned long old, unsigned long new)
+{
+ unsigned long prev;
+
+ __asm__ __volatile__ (
+ PPC_RELEASE_BARRIER
+"1: lwarx %0,0,%2 # __cmpxchg_u32_release\n"
+" cmpw 0,%0,%3\n"
+" bne- 2f\n"
+ PPC405_ERR77(0, %2)
+" stwcx. %4,0,%2\n"
+" bne- 1b\n"
+ "\n"
+"2:"
+ : "=&r" (prev), "+m" (*p)
+ : "r" (p), "r" (old), "r" (new)
+ : "cc", "memory");
+
+ return prev;
+}
+
+
#ifdef CONFIG_PPC64
static __always_inline unsigned long
__cmpxchg_u64(volatile unsigned long *p, unsigned long old, unsigned long new)
@@ -397,6 +422,27 @@ __cmpxchg_u64_acquire(u64 *p, unsigned long old, unsigned long new)

return prev;
}
+
+static __always_inline unsigned long
+__cmpxchg_u64_release(u64 *p, unsigned long old, unsigned long new)
+{
+ unsigned long prev;
+
+ __asm__ __volatile__ (
+ PPC_RELEASE_BARRIER
+"1: ldarx %0,0,%2 # __cmpxchg_u64_release\n"
+" cmpd 0,%0,%3\n"
+" bne- 2f\n"
+" stdcx. %4,0,%2\n"
+" bne- 1b\n"
+ "\n"
+"2:"
+ : "=&r" (prev), "+m" (*p)
+ : "r" (p), "r" (old), "r" (new)
+ : "cc", "memory");
+
+ return prev;
+}
#endif

static __always_inline unsigned long
@@ -478,6 +524,27 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
BUILD_BUG_ON_MSG(1, "Unsupported size for __cmpxchg_acquire");
return old;
}
+
+static __always_inline unsigned long
+__cmpxchg_release(void *ptr, unsigned long old, unsigned long new,
+ unsigned int size)
+{
+ switch (size) {
+ case 1:
+ return __cmpxchg_u8_release(ptr, old, new);
+ case 2:
+ return __cmpxchg_u16_release(ptr, old, new);
+ case 4:
+ return __cmpxchg_u32_release(ptr, old, new);
+#ifdef CONFIG_PPC64
+ case 8:
+ return __cmpxchg_u64_release(ptr, old, new);
+#endif
+ }
+ BUILD_BUG_ON_MSG(1, "Unsupported size for __cmpxchg_release");
+ return old;
+}
+
#define cmpxchg(ptr, o, n) \
({ \
__typeof__(*(ptr)) _o_ = (o); \
@@ -512,6 +579,15 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
(unsigned long)_o_, (unsigned long)_n_, \
sizeof(*(ptr))); \
})
+
+#define cmpxchg_release(ptr, o, n) \
+({ \
+ __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _n_ = (n); \
+ (__typeof__(*(ptr))) __cmpxchg_release((ptr), \
+ (unsigned long)_o_, (unsigned long)_n_, \
+ sizeof(*(ptr))); \
+})
#ifdef CONFIG_PPC64
#define cmpxchg64(ptr, o, n) \
({ \
@@ -533,6 +609,11 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
BUILD_BUG_ON(sizeof(*(ptr)) != 8); \
cmpxchg_acquire((ptr), (o), (n)); \
})
+#define cmpxchg64_release(ptr, o, n) \
+({ \
+ BUILD_BUG_ON(sizeof(*(ptr)) != 8); \
+ cmpxchg_release((ptr), (o), (n)); \
+})
#else
#include <asm-generic/cmpxchg-local.h>
#define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
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