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Message-Id: <20090115103406.a85968f0.akpm@linux-foundation.org>
Date: Thu, 15 Jan 2009 10:34:06 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ingo Molnar <mingo@...e.hu>
Cc: Tejun Heo <tj@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
Brian Gerst <brgerst@...il.com>, ebiederm@...ssion.com,
cl@...ux-foundation.org, rusty@...tcorp.com.au, travis@....com,
linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
On Thu, 15 Jan 2009 19:02:59 +0100 Ingo Molnar <mingo@...e.hu> wrote:
>
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> > On Thu, 15 Jan 2009 22:23:19 +0900 Tejun Heo <tj@...nel.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.
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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