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SubjectRe: [PATCH V2 11/19] csky: Atomic operations
On Thu, Jul 05, 2018 at 07:50:59PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 02, 2018 at 01:30:14AM +0800, Guo Ren wrote:
>
> > +#include <asm/barrier.h>
> > +
> > +#define __xchg(new, ptr, size) \
> > +({ \
> > + __typeof__(ptr) __ptr = (ptr); \
> > + __typeof__(new) __new = (new); \
> > + __typeof__(*(ptr)) __ret; \
> > + unsigned long tmp; \
> > + switch (size) { \
> > + case 4: \
> > + asm volatile ( \
> > + "1: ldex.w %0, (%3) \n" \
> > + " mov %1, %2 \n" \
> > + " stex.w %1, (%3) \n" \
> > + " bez %1, 1b \n" \
> > + : "=&r" (__ret), "=&r" (tmp) \
> > + : "r" (__new), "r"(__ptr) \
> > + : "memory"); \
> > + smp_mb(); \
> > + break; \
> > + default: \
> > + BUILD_BUG(); \
> > + } \
> > + __ret; \
> > +})
> > +
> > +#define xchg(ptr, x) (__xchg((x), (ptr), sizeof(*(ptr))))
> > +
> > +#define __cmpxchg(ptr, old, new, size) \
> > +({ \
> > + __typeof__(ptr) __ptr = (ptr); \
> > + __typeof__(new) __new = (new); \
> > + __typeof__(new) __tmp; \
> > + __typeof__(old) __old = (old); \
> > + __typeof__(*(ptr)) __ret; \
> > + switch (size) { \
> > + case 4: \
> > + asm volatile ( \
> > + "1: ldex.w %0, (%3) \n" \
> > + " cmpne %0, %4 \n" \
> > + " bt 2f \n" \
> > + " mov %1, %2 \n" \
> > + " stex.w %1, (%3) \n" \
> > + " bez %1, 1b \n" \
> > + "2: \n" \
> > + : "=&r" (__ret), "=&r" (__tmp) \
> > + : "r" (__new), "r"(__ptr), "r"(__old) \
> > + : "memory"); \
> > + smp_mb(); \
> > + break; \
> > + default: \
> > + BUILD_BUG(); \
> > + } \
> > + __ret; \
> > +})
> > +
> > +#define cmpxchg(ptr, o, n) \
> > + (__cmpxchg((ptr), (o), (n), sizeof(*(ptr))))
>
> What's the memory ordering rules for your LDEX/STEX ?
Every CPU has a local exclusive monitor.

"Ldex rz, (rx, #off)" will add an entry into the local monitor, and the
entry is composed of a address tag and a exclusive flag (inited with 1).
Any stores (include other cores') will break the exclusive flag to 0 in
the entry which could be indexed by the address tag.

"Stex rz, (rx, #off)" has two condition:
1. Store Success: When the entry's exclusive flag is 1, it will store rz
to the [rx + off] address and the rz will be set to 1.
2. Store Failure: When the entry's exclusive flag is 0, just rz will be
set to 0.

> The mandated semantics for xchg() / cmpxchg() is an effective smp_mb()
> before _and_ after.

switch (size) { \
case 4: \
smp_mb(); \
asm volatile ( \
"1: ldex.w %0, (%3) \n" \
" mov %1, %2 \n" \
" stex.w %1, (%3) \n" \
" bez %1, 1b \n" \
: "=&r" (__ret), "=&r" (tmp) \
: "r" (__new), "r"(__ptr) \
: "memory"); \
smp_mb(); \
break; \
Hmm?
But I couldn't undertand what's wrong without the 1th smp_mb()?
1th smp_mb will make all ld/st finish before ldex.w. Is it necessary?

> The above implementation suggests LDEX implies a SYNC.IS, is this
> correct?
No, ldex doesn't imply a sync.is.

Guo Ren

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