linux-kernel - Re: [PATCH] percpu_counter: change inaccurate comment

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Date:	Thu, 07 Oct 2010 22:42:11 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu_counter: change inaccurate comment

Le jeudi 07 octobre 2010 à 15:13 -0500, Christoph Lameter a écrit :
> On Thu, 7 Oct 2010, Eric Dumazet wrote:
> 
> > If enclosed by preempt_disable()/preempt_enable(), maybe we could use
> > __this_cpu_ptr() ?
> 
> The only difference between __this_cpu_ptr and this_cpu_ptr is that
> this_cpu_ptr checks that preempt was disabled. __this_cpu_ptr allows use
> even without preempt. Preempt must be disabled here so the use of
> this_cpu_ptr is appropriate.
> 
> 

Thats not how I read the thing.

In both variants, preemption _must_ be disabled, its only the context
that can tell how sure we are...

<quote>

commit 7340a0b15280c

__this_cpu_ptr  -> Do not check for preemption context
this_cpu_ptr    -> Check preemption context

</quote>

If preemption was enabled, both pointers would not be very useful...

We use __this_cpu_ptr() in contexts where cpu _cannot_ change under us,
(we just disabled preemption one line above), so its not necessary to
perform the check.

vi +316 include/linux/percpu.h

#define _this_cpu_generic_to_op(pcp, val, op)                   \
do {                                                            \
        preempt_disable();                                      \
        *__this_cpu_ptr(&(pcp)) op val;                         \
        preempt_enable();                                       \
} while (0)

...

#define __this_cpu_generic_to_op(pcp, val, op)                  \
do {                                                            \
        *__this_cpu_ptr(&(pcp)) op val;                         \
} while (0)





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