Re: [PATCH] percpu: add optimized generic percpu accessors

[Andrew Morton]

On Thu, 15 Jan 2009 19:02:59 +0100 Ingo Molnar <mingo@xxxxxxx> wrote:

> 
> * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, 15 Jan 2009 22:23:19 +0900 Tejun Heo <tj@xxxxxxxxxx> wrote:
> > 
> > > --- a/include/asm-generic/percpu.h
> > > +++ b/include/asm-generic/percpu.h
> > > @@ -80,4 +80,56 @@ extern void setup_per_cpu_areas(void);
> > >  #define DECLARE_PER_CPU(type, name) extern PER_CPU_ATTRIBUTES \
> > >  					__typeof__(type) per_cpu_var(name)
> > >  
> > > +/*
> > > + * Optional methods for optimized non-lvalue per-cpu variable access.
> > > + *
> > > + * @var can be a percpu variable or a field of it and its size should
> > > + * equal char, int or long.  percpu_read() evaluates to a lvalue and
> > > + * all others to void.
> > > + *
> > > + * These operations are guaranteed to be atomic w.r.t. preemption.
> > > + * The generic versions use plain get/put_cpu_var().  Archs are
> > > + * encouraged to implement single-instruction alternatives which don't
> > > + * require preemption protection.
> > > + */
> > > +#ifndef percpu_read
> > > +# define percpu_read(var)						\
> > > +  ({									\
> > > +	typeof(per_cpu_var(var)) __tmp_var__;				\
> > > +	__tmp_var__ = get_cpu_var(var);					\
> > > +	put_cpu_var(var);						\
> > > +	__tmp_var__;							\
> > > +  })
> > > +#endif
> > 
> > I wonder if the preempt_disable()/preempt_enable() in here actually
> > does anything useful on any architecture.
> 
> Provides "this is IRQ safe"

?

> and "this is preempt safe" semantics.

Of course.  But do any architectures actually _need_ that for a single
read?

Maybe.  And if so, they can interpose their arch-specific
implementation.  But if the generic version is optimal for them, they
wouldn't need to..


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