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Subject[PATCH] locking/atomics/powerpc: Move cmpxchg helpers to asm/cmpxchg.h and define the full set of cmpxchg APIs

* Boqun Feng <boqun.feng@gmail.com> wrote:

> > May I suggest the patch below? No change in functionality, but it documents the
> > lack of the cmpxchg_release() APIs and maps them explicitly to the full cmpxchg()
> > version. (Which the generic code does now in a rather roundabout way.)
> >
>
> Hmm.. cmpxchg_release() is actually lwsync() + cmpxchg_relaxed(), but
> you just make it sync() + cmpxchg_relaxed() + sync() with the fallback,
> and sync() is much heavier, so I don't think the fallback is correct.

Indeed!

The bit I missed previously is that PowerPC provides its own __atomic_op_release()
method:

#define __atomic_op_release(op, args...) \
({ \
__asm__ __volatile__(PPC_RELEASE_BARRIER "" : : : "memory"); \
op##_relaxed(args); \
})

... which maps to LWSYNC as you say, and my patch made that worse.

> I think maybe you can move powerpc's __atomic_op_{acqurie,release}()
> from atomic.h to cmpxchg.h (in arch/powerpc/include/asm), and
>
> #define cmpxchg_release __atomic_op_release(cmpxchg, __VA_ARGS__);
> #define cmpxchg64_release __atomic_op_release(cmpxchg64, __VA_ARGS__);
>
> I put a diff below to say what I mean (untested).
>
> > Also, the change to arch/powerpc/include/asm/atomic.h has no functional effect
> > right now either, but should anyone add a _relaxed() variant in the future, with
> > this change atomic_cmpxchg_release() and atomic64_cmpxchg_release() will pick that
> > up automatically.
> >
>
> You mean with your other modification in include/linux/atomic.h, right?
> Because with the unmodified include/linux/atomic.h, we already pick that
> autmatically. If so, I think that's fine.
>
> Here is the diff for the modification for cmpxchg_release(), the idea is
> we generate them in asm/cmpxchg.h other than linux/atomic.h for ppc, so
> we keep the new linux/atomic.h working. Because if I understand
> correctly, the next linux/atomic.h only accepts that
>
> 1) architecture only defines fully ordered primitives
>
> or
>
> 2) architecture only defines _relaxed primitives
>
> or
>
> 3) architecture defines all four (fully, _relaxed, _acquire,
> _release) primitives
>
> So powerpc needs to define all four primitives in its only
> asm/cmpxchg.h.

Correct, although the new logic is still RFC, PeterZ didn't like the first version
I proposed and might NAK them.

Thanks for the patch - I have created the patch below from it and added your
Signed-off-by.

The only change I made beyond a trivial build fix is that I also added the release
atomics variants explicitly:

+#define atomic_cmpxchg_release(v, o, n) \
+ cmpxchg_release(&((v)->counter), (o), (n))
+#define atomic64_cmpxchg_release(v, o, n) \
+ cmpxchg_release(&((v)->counter), (o), (n))

It has passed a PowerPC cross-build test here, but no runtime tests.

Does this patch look good to you?

(Still subject to PeterZ's Ack/NAK.)

Thanks,

Ingo

======================>
From: Boqun Feng <boqun.feng@gmail.com>
Date: Sat, 5 May 2018 19:28:17 +0800
Subject: [PATCH] locking/atomics/powerpc: Move cmpxchg helpers to asm/cmpxchg.h and define the full set of cmpxchg APIs

Move PowerPC's __op_{acqurie,release}() from atomic.h to
cmpxchg.h (in arch/powerpc/include/asm), plus use them to
define these two methods:

#define cmpxchg_release __op_release(cmpxchg, __VA_ARGS__);
#define cmpxchg64_release __op_release(cmpxchg64, __VA_ARGS__);

... the idea is to generate all these methods in cmpxchg.h and to define the full
array of atomic primitives, including the cmpxchg_release() methods which were
defined by the generic code before.

Also define the atomic[64]_() variants explicitly.

This ensures that all these low level cmpxchg APIs are defined in
PowerPC headers, with no generic header fallbacks.

No change in functionality or code generation.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: aryabinin@virtuozzo.com
Cc: catalin.marinas@arm.com
Cc: dvyukov@google.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/20180505112817.ihrb726i37bwm4cj@tardis
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/powerpc/include/asm/atomic.h | 22 ++++------------------
arch/powerpc/include/asm/cmpxchg.h | 24 ++++++++++++++++++++++++
2 files changed, 28 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/include/asm/atomic.h b/arch/powerpc/include/asm/atomic.h
index 682b3e6a1e21..4e06955ec10f 100644
--- a/arch/powerpc/include/asm/atomic.h
+++ b/arch/powerpc/include/asm/atomic.h
@@ -13,24 +13,6 @@

#define ATOMIC_INIT(i) { (i) }

-/*
- * Since *_return_relaxed and {cmp}xchg_relaxed are implemented with
- * a "bne-" instruction at the end, so an isync is enough as a acquire barrier
- * on the platform without lwsync.
- */
-#define __atomic_op_acquire(op, args...) \
-({ \
- typeof(op##_relaxed(args)) __ret = op##_relaxed(args); \
- __asm__ __volatile__(PPC_ACQUIRE_BARRIER "" : : : "memory"); \
- __ret; \
-})
-
-#define __atomic_op_release(op, args...) \
-({ \
- __asm__ __volatile__(PPC_RELEASE_BARRIER "" : : : "memory"); \
- op##_relaxed(args); \
-})
-
static __inline__ int atomic_read(const atomic_t *v)
{
int t;
@@ -213,6 +195,8 @@ 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 +503,8 @@ 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..e27a612b957f 100644
--- a/arch/powerpc/include/asm/cmpxchg.h
+++ b/arch/powerpc/include/asm/cmpxchg.h
@@ -8,6 +8,24 @@
#include <asm/asm-compat.h>
#include <linux/bug.h>

+/*
+ * Since *_return_relaxed and {cmp}xchg_relaxed are implemented with
+ * a "bne-" instruction at the end, so an isync is enough as a acquire barrier
+ * on the platform without lwsync.
+ */
+#define __atomic_op_acquire(op, args...) \
+({ \
+ typeof(op##_relaxed(args)) __ret = op##_relaxed(args); \
+ __asm__ __volatile__(PPC_ACQUIRE_BARRIER "" : : : "memory"); \
+ __ret; \
+})
+
+#define __atomic_op_release(op, args...) \
+({ \
+ __asm__ __volatile__(PPC_RELEASE_BARRIER "" : : : "memory"); \
+ op##_relaxed(args); \
+})
+
#ifdef __BIG_ENDIAN
#define BITOFF_CAL(size, off) ((sizeof(u32) - size - off) * BITS_PER_BYTE)
#else
@@ -512,6 +530,9 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
(unsigned long)_o_, (unsigned long)_n_, \
sizeof(*(ptr))); \
})
+
+#define cmpxchg_release(...) __atomic_op_release(cmpxchg, __VA_ARGS__)
+
#ifdef CONFIG_PPC64
#define cmpxchg64(ptr, o, n) \
({ \
@@ -533,6 +554,9 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
BUILD_BUG_ON(sizeof(*(ptr)) != 8); \
cmpxchg_acquire((ptr), (o), (n)); \
})
+
+#define cmpxchg64_release(...) __atomic_op_release(cmpxchg64, __VA_ARGS__)
+
#else
#include <asm-generic/cmpxchg-local.h>
#define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
\
 
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