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Date: Mon, 6 Feb 2017 14:32:15 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: elena.reshetova@...el.com, gregkh@...uxfoundation.org,
keescook@...omium.org, arnd@...db.de, tglx@...utronix.de,
mingo@...nel.org, h.peter.anvin@...el.com, will.deacon@....com,
dwindsor@...il.com, dhowells@...hat.com,
linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 4/5] atomic: Introduce atomic_try_cmpxchg()
On Mon, Feb 06, 2017 at 12:24:28PM +0800, Boqun Feng wrote:
> On Fri, Feb 03, 2017 at 02:26:02PM +0100, Peter Zijlstra wrote:
> > Add a new cmpxchg interface:
> >
> > bool try_cmpxchg(u{8,16,32,64} *ptr, u{8,16,32,64} *val, u{8,16,32,64} new);
> >
> > Where the boolean returns the result of the compare; and thus if the
> > exchange happened; and in case of failure, the new value of *ptr is
> > returned in *val.
> >
> > This allows simplification/improvement of loops like:
> >
> > for (;;) {
> > new = val $op $imm;
> > old = cmpxchg(ptr, val, new);
> > if (old == val)
> > break;
> > val = old;
> > }
> >
> > into:
> >
> > for (;;) {
> > new = val $op $imm;
> > if (try_cmpxchg(ptr, &val, new))
> > break;
> > }
> >
> > while also generating better code (GCC6 and onwards).
> >
>
> But switching to try_cmpxchg() will make @val a memory location, which
> could not be put in a register. And this will generate unnecessary
> memory accesses on archs having enough registers(PPC, e.g.).
>
Hmm.. it turns out that compilers can figure out that @val could be fit
in a register(maybe in the escape analysis step), so don't treat it as a
memory location. This is at least true for GCC 5.4.0 on PPC. So I think
we can rely on this?
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > --- a/arch/x86/include/asm/atomic.h
> > +++ b/arch/x86/include/asm/atomic.h
> > @@ -186,6 +186,12 @@ static __always_inline int atomic_cmpxch
> > return cmpxchg(&v->counter, old, new);
> > }
> >
> > +#define atomic_try_cmpxchg atomic_try_cmpxchg
> > +static __always_inline bool atomic_try_cmpxchg(atomic_t *v, int *old, int new)
> > +{
> > + return try_cmpxchg(&v->counter, old, new);
> > +}
> > +
> > static inline int atomic_xchg(atomic_t *v, int new)
> > {
> > return xchg(&v->counter, new);
> > --- a/arch/x86/include/asm/cmpxchg.h
> > +++ b/arch/x86/include/asm/cmpxchg.h
> > @@ -153,6 +153,75 @@ extern void __add_wrong_size(void)
> > #define cmpxchg_local(ptr, old, new) \
> > __cmpxchg_local(ptr, old, new, sizeof(*(ptr)))
> >
> > +
> > +#define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \
> > +({ \
> > + bool success; \
> > + __typeof__(_ptr) _old = (_pold); \
> > + __typeof__(*(_ptr)) __old = *_old; \
> > + __typeof__(*(_ptr)) __new = (_new); \
> > + switch (size) { \
> > + case __X86_CASE_B: \
> > + { \
> > + volatile u8 *__ptr = (volatile u8 *)(_ptr); \
> > + asm volatile(lock "cmpxchgb %[new], %[ptr]" \
> > + CC_SET(z) \
> > + : CC_OUT(z) (success), \
> > + [ptr] "+m" (*__ptr), \
> > + [old] "+a" (__old) \
> > + : [new] "q" (__new) \
> > + : "memory"); \
> > + break; \
> > + } \
> > + case __X86_CASE_W: \
> > + { \
> > + volatile u16 *__ptr = (volatile u16 *)(_ptr); \
> > + asm volatile(lock "cmpxchgw %[new], %[ptr]" \
> > + CC_SET(z) \
> > + : CC_OUT(z) (success), \
> > + [ptr] "+m" (*__ptr), \
> > + [old] "+a" (__old) \
> > + : [new] "r" (__new) \
> > + : "memory"); \
> > + break; \
> > + } \
> > + case __X86_CASE_L: \
> > + { \
> > + volatile u32 *__ptr = (volatile u32 *)(_ptr); \
> > + asm volatile(lock "cmpxchgl %[new], %[ptr]" \
> > + CC_SET(z) \
> > + : CC_OUT(z) (success), \
> > + [ptr] "+m" (*__ptr), \
> > + [old] "+a" (__old) \
> > + : [new] "r" (__new) \
> > + : "memory"); \
> > + break; \
> > + } \
> > + case __X86_CASE_Q: \
> > + { \
> > + volatile u64 *__ptr = (volatile u64 *)(_ptr); \
> > + asm volatile(lock "cmpxchgq %[new], %[ptr]" \
> > + CC_SET(z) \
> > + : CC_OUT(z) (success), \
> > + [ptr] "+m" (*__ptr), \
> > + [old] "+a" (__old) \
> > + : [new] "r" (__new) \
> > + : "memory"); \
> > + break; \
> > + } \
> > + default: \
> > + __cmpxchg_wrong_size(); \
> > + } \
> > + *_old = __old; \
> > + success; \
> > +})
> > +
> > +#define __try_cmpxchg(ptr, pold, new, size) \
> > + __raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX)
> > +
> > +#define try_cmpxchg(ptr, pold, new) \
> > + __try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr)))
> > +
> > /*
> > * xadd() adds "inc" to "*ptr" and atomically returns the previous
> > * value of "*ptr".
> > --- a/include/linux/atomic.h
> > +++ b/include/linux/atomic.h
> > @@ -423,6 +423,28 @@
> > #endif
> > #endif /* atomic_cmpxchg_relaxed */
> >
> > +#ifndef atomic_try_cmpxchg
> > +
> > +#define __atomic_try_cmpxchg(type, _p, _po, _n) \
> > +({ \
> > + typeof(_po) __po = (_po); \
> > + typeof(*(_po)) __o = *__po; \
> > + bool success = (atomic_cmpxchg##type((_p), __o, (_n)) == __o); \
> > + *__po = __o; \
>
> Besides, is this part correct? atomic_cmpxchg_*() wouldn't change the
> value of __o, so *__po wouldn't be changed.. IOW, in case of failure,
> *ptr wouldn't be updated to a new value.
>
> Maybe this should be:
>
> bool success;
> *__po = atomic_cmpxchg##type((_p), __o, (_n));
> sucess = (*__po == _o);
typo... should be
success = (*__po == __o);
Regards,
Boqun
>
> , right?
>
> Regards,
> Boqun
>
> > + success; \
> > +})
> > +
> > +#define atomic_try_cmpxchg(_p, _po, _n) __atomic_try_cmpxchg(, _p, _po, _n)
> > +#define atomic_try_cmpxchg_relaxed(_p, _po, _n) __atomic_try_cmpxchg(_relaxed, _p, _po, _n)
> > +#define atomic_try_cmpxchg_acquire(_p, _po, _n) __atomic_try_cmpxchg(_acquire, _p, _po, _n)
> > +#define atomic_try_cmpxchg_release(_p, _po, _n) __atomic_try_cmpxchg(_release, _p, _po, _n)
> > +
> > +#else /* atomic_try_cmpxchg */
> > +#define atomic_try_cmpxchg_relaxed atomic_try_cmpxchg
> > +#define atomic_try_cmpxchg_acquire atomic_try_cmpxchg
> > +#define atomic_try_cmpxchg_release atomic_try_cmpxchg
> > +#endif /* atomic_try_cmpxchg */
> > +
> > /* cmpxchg_relaxed */
> > #ifndef cmpxchg_relaxed
> > #define cmpxchg_relaxed cmpxchg
> >
> >
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