linux-kernel - Re: per cpun+ spin locks coexistence?

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Message-ID: <804dabb00803171006i4423c5b6w58bd95116510d3f5@mail.gmail.com>
Date:	Tue, 18 Mar 2008 01:06:33 +0800
From:	"Peter Teoh" <htmldeveloper@...il.com>
To:	"Johannes Weiner" <hannes@...urebad.de>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Tejun Heo" <htejun@...il.com>,
	"Dipankar Sarma" <dipankar@...ibm.com>
Subject: Re: per cpun+ spin locks coexistence?

Thanks for the explanation, much apologies for this newbie discussion.
  But I still find it inexplicable:

On Mon, Mar 17, 2008 at 4:20 AM, Johannes Weiner <hannes@...urebad.de> wrote:
>
>  A per-cpu variable is basically an array the size of the number of
>  possible CPUs in the system.  get_cpu_var() checks what current CPU we
>  are running on and gets the array-element corresponding to this CPU.
>
>  So, really oversimplified, get_cpu_var(foo) translates to something like
>  foo[smp_processor_id()].
>

Ok, so calling get_cpu_var() always return the array-element for the
current CPU, and since by design, only the current CPU can
modify/write to this array element (this is my assumption - correct?),
and the other CPU will just read it (using the per_cpu construct).
So far correct?   So why do u still need to spin_lock() to lock other
CPU from accessing - the other CPU will always just READ it, so just
go ahead and let them read it.   Seemed like it defeats the purpose of
get_cpu_var()'s design?

But supposed u really want to put a spin_lock(), just to be sure
nobody is even reading it, or modifying it, so then what is the
original purpose of get_cpu_var() - is it not to implement something
that can be parallelized among different CPU, without affecting each
other, and using no locks?

The dual use of spin_lock+get_cpu_var() confuses me here :-).   (not
the per_cpu(), which I agree is supposed to be callabe from all the
different CPU, for purpose of enumeration or data collection).

-- 
Regards,
Peter Teoh
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