Re: [PATCH -rt 3/5] asm/local.h cmpxchg

[Mathieu Desnoyers]

* Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> From: Christoph Lameter <clameter@xxxxxxx>
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  include/asm-generic/local.h |   25 +++++++++++++++++++++++--
>  include/asm-i386/local.h    |   13 +++++++++++++
>  include/asm-x86_64/local.h  |   16 ++++++++++++++++
>  3 files changed, 52 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6.22-rc6-mm1/include/asm-generic/local.h
> ===================================================================
> --- linux-2.6.22-rc6-mm1.orig/include/asm-generic/local.h	2007-07-12 19:44:18.000000000 -0700
> +++ linux-2.6.22-rc6-mm1/include/asm-generic/local.h	2007-07-12 19:44:57.000000000 -0700
> @@ -46,13 +46,34 @@ typedef struct
>  #define local_add_unless(l, a, u) atomic_long_add_unless((&(l)->a), (a), (u))
>  #define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a)
>  
> -/* Non-atomic variants, ie. preemption disabled and won't be touched
> - * in interrupt, etc.  Some archs can optimize this case well. */
> +/*
> + * Establish a state necessary for __local_xx functions to work.
> + */
> +#define __local_begin(flags)	local_irq_disable(flags)
> +
> +static inline void __local_end(unsigned long flags)
> +{
> +	local_irq_restore(flags);
> +}
> +

Wouldn't it be cheaper to use preempt_disable/enable instead of irq
disable/enable in asm-generic to protect the __local accesses since they
are not supposed to be touched by interrupt context ?

I also think that __local_begin/end() should be changed into
local_begin() and local_end(). It makes sense to use this around all
local_t variable accesses.

> +/*
> + * Non-atomic variants, ie. within local_begin() / local_end() or
> + * preempt_disable / enable() and won't be touched in interrupt, etc.
> + * Some archs can optimize this case well.
> + */
>  #define __local_inc(l)		local_set((l), local_read(l) + 1)
>  #define __local_dec(l)		local_set((l), local_read(l) - 1)
>  #define __local_add(i,l)	local_set((l), local_read(l) + (i))
>  #define __local_sub(i,l)	local_set((l), local_read(l) - (i))
>  
> +#define __local_cmpxchg((v), (o), (n)) (*(v) = (n), (o))

Shouldn't this look like a while loop instead ? Where is the
comparison ? It should use local_set and local_read...

The proper way to do this would be to take all architectures that only
define a atomic_cmpxchg and atomic_xchg and turn that into a cmpxchg and
xchg. Then, the same could be done for architectures which only have a
local_cmpxchg, but no cmpxchg_local.

Then, cmpxchg_local could be used to touch a variable in an atomic wrt
cpu fashion without wrapping the variable in a local_t.

All the local_*() functions should only touch local_t types.

If local_begin()/local_end() is defined as preempt disable/enable, then
it's ok to use this to protect __local_*() accesses. However, if you
turn this into a migrate_disable/enable(), then the protection against
other threads on the same CPU is not insured.

> +#define __local_xchg((v), (n)) 						\
> +({									\
> +	__typeof(v) x = *(v);						\
> +	*(v) = (n);							\
> +	x;								\
> +)}
> +
>  /* Use these for per-cpu local_t variables: on some archs they are
>   * much more efficient than these naive implementations.  Note they take
>   * a variable (eg. mystruct.foo), not an address.
> Index: linux-2.6.22-rc6-mm1/include/asm-x86_64/local.h
> ===================================================================
> --- linux-2.6.22-rc6-mm1.orig/include/asm-x86_64/local.h	2007-07-12 19:44:18.000000000 -0700
> +++ linux-2.6.22-rc6-mm1/include/asm-x86_64/local.h	2007-07-12 19:44:57.000000000 -0700
> @@ -9,6 +9,7 @@ typedef struct
>  	atomic_long_t a;
>  } local_t;
>  
> +

Empty line ?

>  #define LOCAL_INIT(i)	{ ATOMIC_LONG_INIT(i) }
>  
>  #define local_read(l)	atomic_long_read(&(l)->a)
> @@ -181,11 +182,26 @@ static __inline__ long local_sub_return(
>  
>  /* On x86-64 these are better than the atomic variants on SMP kernels
>     because they dont use a lock prefix. */
> +
> +#define __local_begin(__flags)		\
> +{					\
> +	(__flags) = 0;			\

What are these flags doing ? And why are they not set back in
__local_end ? I'm a bit curious :)

> +	preempt_disable();		\
> +}
> +
> +static inline void __local_end(unsigned long flags) {
> +	preempt_enable();
> +}
> +

Same here : __local_begin/end() -> local_begin/end().

>  #define __local_inc(l)		local_inc(l)
>  #define __local_dec(l)		local_dec(l)
>  #define __local_add(i,l)	local_add((i),(l))
>  #define __local_sub(i,l)	local_sub((i),(l))
>  
> +#define __local_cmpxchg		cmpxchg_local
> +#define __local_xchg		xchg
> +

local_*() should only touch local_t vars.

> +
>  /* Use these for per-cpu local_t variables: on some archs they are
>   * much more efficient than these naive implementations.  Note they take
>   * a variable, not an address.
> Index: linux-2.6.22-rc6-mm1/include/asm-i386/local.h
> ===================================================================
> --- linux-2.6.22-rc6-mm1.orig/include/asm-i386/local.h	2007-07-12 19:53:00.000000000 -0700
> +++ linux-2.6.22-rc6-mm1/include/asm-i386/local.h	2007-07-12 19:55:22.000000000 -0700
> @@ -194,12 +194,25 @@ static __inline__ long local_sub_return(
>  })
>  #define local_inc_not_zero(l) local_add_unless((l), 1, 0)
>  
> +#define __local_begin(__flags)					\
> +{								\
> +	(__flags) = 0;						\
> +	preempt_disable();					\
> +}
> +
> +static inline void __local_end(unsigned long flags) {
> +	preempt_enable();
> +}

__local_begin/end -> local_begin/end.

> +
>  /* On x86, these are no better than the atomic variants. */
>  #define __local_inc(l)		local_inc(l)
>  #define __local_dec(l)		local_dec(l)
>  #define __local_add(i,l)	local_add((i),(l))
>  #define __local_sub(i,l)	local_sub((i),(l))
>  
> +#define __local_cmpxchg		cmpxchg_local
> +#define __local_xchg		xchg
> +

local_*() should only touch local_t vars.

Mathieu

>  /* Use these for per-cpu local_t variables: on some archs they are
>   * much more efficient than these naive implementations.  Note they take
>   * a variable, not an address.
> 
> --
> 

-- 
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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