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Message-ID: <20070820203807.GB20936@Krystal>
Date: Mon, 20 Aug 2007 16:38:07 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Cc: clameter@....com, mingo@...hat.com
Subject: Re: [patch 01/11] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486
Clearly this signle patch is a duplicate from the previous patchset,
please do not apply.
* Mathieu Desnoyers (mathieu.desnoyers@...ymtl.ca) wrote:
> Actually, on 386, cmpxchg and cmpxchg_local fall back on
> cmpxchg_386_u8/16/32: it disables interruptions around non atomic
> updates to mimic the cmpxchg behavior.
>
> The comment:
> /* Poor man's cmpxchg for 386. Unsuitable for SMP */
>
> already present in cmpxchg_386_u32 tells much about how this cmpxchg
> implementation should not be used in a SMP context. However, the cmpxchg_local
> can perfectly use this fallback, since it only needs to be atomic wrt the local
> cpu.
>
> This patch adds a cmpxchg_486_u64 and uses it as a fallback for cmpxchg64
> and cmpxchg64_local on 80386 and 80486.
>
> Q:
> but why is it called cmpxchg_486 when the other functions are called
>
> A:
> Because the standard cmpxchg is missing only on 386, but cmpxchg8b is
> missing both on 386 and 486.
>
> Citing Intel's Instruction set reference:
>
> cmpxchg:
> This instruction is not supported on Intel processors earlier than the
> Intel486 processors.
>
> cmpxchg8b:
> This instruction encoding is not supported on Intel processors earlier
> than the Pentium processors.
>
> Q:
> What's the reason to have cmpxchg64_local on 32 bit architectures?
> Without that need all this would just be a few simple defines.
>
> A:
> cmpxchg64_local on 32 bits architectures takes unsigned long long
> parameters, but cmpxchg_local only takes longs. Since we have cmpxchg8b
> to execute a 8 byte cmpxchg atomically on pentium and +, it makes sense
> to provide a flavor of cmpxchg and cmpxchg_local using this instruction.
>
> Also, for 32 bits architectures lacking the 64 bits atomic cmpxchg, it
> makes sense _not_ to define cmpxchg64 while cmpxchg could still be
> available.
>
> Moreover, the fallback for cmpxchg8b on i386 for 386 and 486 is a
> different case than cmpxchg (which is only required for 386). Using
> different code makes this easier.
>
> However, cmpxchg64_local will be emulated by disabling interrupts on all
> architectures where it is not supported atomically.
>
> Therefore, we *could* turn cmpxchg64_local into a cmpxchg_local, but it
> would make the 386/486 fallbacks ugly, make its design different from
> cmpxchg/cmpxchg64 (which really depends on atomic operations and cannot
> be emulated) and require the __cmpxchg_local to be expressed as a macro
> rather than an inline function so the parameters would not be fixed to
> unsigned long long in every case.
>
> So I think cmpxchg64_local makes sense there, but I am open to
> suggestions.
>
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> CC: clameter@....com
> CC: mingo@...hat.com
> ---
> arch/i386/kernel/cpu/intel.c | 17 +++++++
> include/asm-i386/cmpxchg.h | 100 +++++++++++++++++++++++++++++--------------
> 2 files changed, 85 insertions(+), 32 deletions(-)
>
> Index: linux-2.6-lttng/arch/i386/kernel/cpu/intel.c
> ===================================================================
> --- linux-2.6-lttng.orig/arch/i386/kernel/cpu/intel.c 2007-08-07 11:00:43.000000000 -0400
> +++ linux-2.6-lttng/arch/i386/kernel/cpu/intel.c 2007-08-07 11:11:07.000000000 -0400
> @@ -329,5 +329,22 @@ unsigned long cmpxchg_386_u32(volatile v
> EXPORT_SYMBOL(cmpxchg_386_u32);
> #endif
>
> +#ifndef CONFIG_X86_CMPXCHG64
> +unsigned long long cmpxchg_486_u64(volatile void *ptr, u64 old, u64 new)
> +{
> + u64 prev;
> + unsigned long flags;
> +
> + /* Poor man's cmpxchg8b for 386 and 486. Unsuitable for SMP */
> + local_irq_save(flags);
> + prev = *(u64 *)ptr;
> + if (prev == old)
> + *(u64 *)ptr = new;
> + local_irq_restore(flags);
> + return prev;
> +}
> +EXPORT_SYMBOL(cmpxchg_486_u64);
> +#endif
> +
> // arch_initcall(intel_cpu_init);
>
> Index: linux-2.6-lttng/include/asm-i386/cmpxchg.h
> ===================================================================
> --- linux-2.6-lttng.orig/include/asm-i386/cmpxchg.h 2007-08-07 11:01:17.000000000 -0400
> +++ linux-2.6-lttng/include/asm-i386/cmpxchg.h 2007-08-07 11:16:42.000000000 -0400
> @@ -116,6 +116,15 @@ static inline unsigned long __xchg(unsig
> (unsigned long)(n),sizeof(*(ptr))))
> #endif
>
> +#ifdef CONFIG_X86_CMPXCHG64
> +#define cmpxchg64(ptr,o,n)\
> + ((__typeof__(*(ptr)))__cmpxchg64((ptr),(unsigned long long)(o),\
> + (unsigned long long)(n)))
> +#define cmpxchg64_local(ptr,o,n)\
> + ((__typeof__(*(ptr)))__cmpxchg64_local((ptr),(unsigned long long)(o),\
> + (unsigned long long)(n)))
> +#endif
> +
> static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
> unsigned long new, int size)
> {
> @@ -203,6 +212,34 @@ static inline unsigned long __cmpxchg_lo
> return old;
> }
>
> +static inline unsigned long long __cmpxchg64(volatile void *ptr,
> + unsigned long long old, unsigned long long new)
> +{
> + unsigned long long prev;
> + __asm__ __volatile__(LOCK_PREFIX "cmpxchg8b %3"
> + : "=A"(prev)
> + : "b"((unsigned long)new),
> + "c"((unsigned long)(new >> 32)),
> + "m"(*__xg(ptr)),
> + "0"(old)
> + : "memory");
> + return prev;
> +}
> +
> +static inline unsigned long long __cmpxchg64_local(volatile void *ptr,
> + unsigned long long old, unsigned long long new)
> +{
> + unsigned long long prev;
> + __asm__ __volatile__("cmpxchg8b %3"
> + : "=A"(prev)
> + : "b"((unsigned long)new),
> + "c"((unsigned long)(new >> 32)),
> + "m"(*__xg(ptr)),
> + "0"(old)
> + : "memory");
> + return prev;
> +}
> +
> #ifndef CONFIG_X86_CMPXCHG
> /*
> * Building a kernel capable running on 80386. It may be necessary to
> @@ -252,38 +289,37 @@ static inline unsigned long cmpxchg_386(
> })
> #endif
>
> -static inline unsigned long long __cmpxchg64(volatile void *ptr, unsigned long long old,
> - unsigned long long new)
> -{
> - unsigned long long prev;
> - __asm__ __volatile__(LOCK_PREFIX "cmpxchg8b %3"
> - : "=A"(prev)
> - : "b"((unsigned long)new),
> - "c"((unsigned long)(new >> 32)),
> - "m"(*__xg(ptr)),
> - "0"(old)
> - : "memory");
> - return prev;
> -}
> +#ifndef CONFIG_X86_CMPXCHG64
> +/*
> + * Building a kernel capable running on 80386 and 80486. It may be necessary
> + * to simulate the cmpxchg8b on the 80386 and 80486 CPU.
> + */
> +
> +extern unsigned long long cmpxchg_486_u64(volatile void *, u64, u64);
> +
> +#define cmpxchg64(ptr,o,n) \
> +({ \
> + __typeof__(*(ptr)) __ret; \
> + if (likely(boot_cpu_data.x86 > 4)) \
> + __ret = __cmpxchg64((ptr), (unsigned long long)(o), \
> + (unsigned long long)(n)); \
> + else \
> + __ret = cmpxchg_486_u64((ptr), (unsigned long long)(o), \
> + (unsigned long long)(n)); \
> + __ret; \
> +})
> +#define cmpxchg64_local(ptr,o,n) \
> +({ \
> + __typeof__(*(ptr)) __ret; \
> + if (likely(boot_cpu_data.x86 > 4)) \
> + __ret = __cmpxchg64_local((ptr), (unsigned long long)(o), \
> + (unsigned long long)(n)); \
> + else \
> + __ret = cmpxchg_486_u64((ptr), (unsigned long long)(o), \
> + (unsigned long long)(n)); \
> + __ret; \
> +})
>
> -static inline unsigned long long __cmpxchg64_local(volatile void *ptr,
> - unsigned long long old, unsigned long long new)
> -{
> - unsigned long long prev;
> - __asm__ __volatile__("cmpxchg8b %3"
> - : "=A"(prev)
> - : "b"((unsigned long)new),
> - "c"((unsigned long)(new >> 32)),
> - "m"(*__xg(ptr)),
> - "0"(old)
> - : "memory");
> - return prev;
> -}
> +#endif
>
> -#define cmpxchg64(ptr,o,n)\
> - ((__typeof__(*(ptr)))__cmpxchg64((ptr),(unsigned long long)(o),\
> - (unsigned long long)(n)))
> -#define cmpxchg64_local(ptr,o,n)\
> - ((__typeof__(*(ptr)))__cmpxchg64_local((ptr),(unsigned long long)(o),\
> - (unsigned long long)(n)))
> #endif
>
> --
> Mathieu Desnoyers
> Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
> -
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>
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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