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

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Thu, 15 Jan 2009 22:23:19 +0900 Tejun Heo <tj@kernel.org> 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.

Ingo


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