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Date: Fri, 13 May 2022 12:20:57 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>,
Mark Rutland <mark.rutland@....com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] locking/atomic/x86: Introduce try_cmpxchg64
On Fri, May 13, 2022 at 11:10 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, May 10, 2022 at 05:42:17PM +0200, Uros Bizjak wrote:
>
> For the Changelog I would focus on the 64bit improvement and leave 32bit
> as a side-note.
Thanks, I will rephrase the ChangeLog.
>
> > ---
> > arch/x86/include/asm/cmpxchg_32.h | 43 ++++++++++++++++++++++
> > arch/x86/include/asm/cmpxchg_64.h | 6 +++
> > include/linux/atomic/atomic-instrumented.h | 40 +++++++++++++++++++-
> > scripts/atomic/gen-atomic-instrumented.sh | 2 +-
> > 4 files changed, 89 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
> > index 0a7fe0321613..e874ff7f7529 100644
> > --- a/arch/x86/include/asm/cmpxchg_32.h
> > +++ b/arch/x86/include/asm/cmpxchg_32.h
> > @@ -42,6 +42,9 @@ static inline void set_64bit(volatile u64 *ptr, u64 value)
> > #define arch_cmpxchg64_local(ptr, o, n) \
> > ((__typeof__(*(ptr)))__cmpxchg64_local((ptr), (unsigned long long)(o), \
> > (unsigned long long)(n)))
> > +#define arch_try_cmpxchg64(ptr, po, n) \
> > + ((__typeof__(*(ptr)))__try_cmpxchg64((ptr), (unsigned long long *)(po), \
> > + (unsigned long long)(n)))
> > #endif
> >
> > static inline u64 __cmpxchg64(volatile u64 *ptr, u64 old, u64 new)
> > @@ -70,6 +73,25 @@ static inline u64 __cmpxchg64_local(volatile u64 *ptr, u64 old, u64 new)
> > return prev;
> > }
> >
> > +static inline bool __try_cmpxchg64(volatile u64 *ptr, u64 *pold, u64 new)
> > +{
> > + bool success;
> > + u64 prev;
> > + asm volatile(LOCK_PREFIX "cmpxchg8b %2"
> > + CC_SET(z)
> > + : CC_OUT(z) (success),
> > + "=A" (prev),
> > + "+m" (*ptr)
> > + : "b" ((u32)new),
> > + "c" ((u32)(new >> 32)),
> > + "1" (*pold)
> > + : "memory");
> > +
> > + if (unlikely(!success))
> > + *pold = prev;
>
> I would prefer this be more like the existing try_cmpxchg code,
> perhaps:
>
> u64 old = *pold;
>
> asm volatile (LOCK_PREFIX "cmpxchg8b %[ptr]"
> CC_SET(z)
> : CC_OUT(z) (success),
> [ptr] "+m" (*ptr)
> "+A" (old)
> : "b" ((u32)new)
> "c" ((u32)(new >> 32))
> : "memory");
>
> if (unlikely(!success))
> *pold = old;
>
> The existing 32bit cmpxchg code is a 'bit' crusty.
I was trying to follow the existing __cmpxchg64 as much as possible,
with the intention of a follow-up patch that would modernize
everything in cmpxchg_32.h. I can surely go the other way and submit
modernized new code.
> > + return success;
> > +}
> > +
> > #ifndef CONFIG_X86_CMPXCHG64
> > /*
> > * Building a kernel capable running on 80386 and 80486. It may be necessary
> > @@ -108,6 +130,27 @@ static inline u64 __cmpxchg64_local(volatile u64 *ptr, u64 old, u64 new)
> > : "memory"); \
> > __ret; })
> >
> > +#define arch_try_cmpxchg64(ptr, po, n) \
> > +({ \
> > + bool success; \
> > + __typeof__(*(ptr)) __prev; \
> > + __typeof__(ptr) _old = (__typeof__(ptr))(po); \
> > + __typeof__(*(ptr)) __old = *_old; \
> > + __typeof__(*(ptr)) __new = (n); \
> > + alternative_io(LOCK_PREFIX_HERE \
> > + "call cmpxchg8b_emu", \
> > + "lock; cmpxchg8b (%%esi)" , \
> > + X86_FEATURE_CX8, \
> > + "=A" (__prev), \
> > + "S" ((ptr)), "0" (__old), \
> > + "b" ((unsigned int)__new), \
> > + "c" ((unsigned int)(__new>>32)) \
> > + : "memory"); \
> > + success = (__prev == __old); \
> > + if (unlikely(!success)) \
> > + *_old = __prev; \
> > + likely(success); \
> > +})
>
> Wouldn't this be better written like the normal fallback wrapper?
>
> static __always_inline bool
> arch_try_cmpxchg64(u64 *v, u64 *old, u64 new)
> {
> u64 r, o = *old;
> r = arch_cmpxchg64(v, o, new);
> if (unlikely(r != o))
> *old = r;
> return likely(r == o);
> }
>
> Less magical, same exact code.
Also, I tried to follow up the existing #defines. Will improve the
code according to your suggestion here.
Thanks,
Uros.
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