linux-kernel - Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

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Date:	Tue, 10 Jun 2008 09:27:48 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

On Tuesday 10 June 2008 05:00:36 Christoph Lameter wrote:
> On Fri, 6 Jun 2008, Rusty Russell wrote:
> > > Also, the above cpu_local_wrap(...) adds:
> > >
> > > 	#define cpu_local_wrap(l)               \
> > > 	({                                      \
> > > 	        preempt_disable();              \
> > > 	        (l);                            \
> > > 	        preempt_enable();               \
> > > 	})                                      \
> > >
> > > ... and there isn't a non-preemption version that I can find.
> >
> > Yes, this should be fixed.  I thought i386 had optimized versions
> > pre-merge, but I was wrong (%gs for per-cpu came later, and noone cleaned
> > up these naive versions).  Did you want me to write them?
>
> How can that be fixed? You have no atomic instruction that calculates the
> per cpu address in one go.

Huh?  "incl %fs:varname" does exactly this.

> And as long as that is the case you need to 
> disable preempt. Otherwise you may increment the per cpu variable of
> another processor because the process was rescheduled after the address
> was calculated but before the increment was done.

But of course, that is not a problem.  You make local_t an atomic_t, and then 
it doesn't matter which CPU you incremented.

By definition if the caller cared, they would have had premption disabled.

Hope that clarifies,
Rusty.
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