linux-kernel - Re: [patch v3 2/3] percpu_counter: fix code for 32bit systems for UP

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Date:	Thu, 14 Apr 2011 10:44:04 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tj@...nel.org" <tj@...nel.org>, "cl@...ux.com" <cl@...ux.com>
Subject: Re: [patch v3 2/3] percpu_counter: fix code for 32bit systems for
 UP

On Thu, 2011-04-14 at 10:35 +0800, Eric Dumazet wrote:
> Le jeudi 14 avril 2011 à 10:04 +0800, shaohua.li@...el.com a écrit :
> > pièce jointe document texte brut (percpu-counter-32bits.patch)
> > percpu_counter.counter is a 's64'. Accessing it in 32-bit system is racing.
> > we need some locking to protect it otherwise some very wrong value could be
> > accessed.
> > 
> > Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> > ---
> >  include/linux/percpu_counter.h |   37 ++++++++++++++++++++++++++-----------
> >  1 file changed, 26 insertions(+), 11 deletions(-)
> > 
> > Index: linux/include/linux/percpu_counter.h
> > ===================================================================
> > --- linux.orig/include/linux/percpu_counter.h	2011-04-14 08:50:45.000000000 +0800
> > +++ linux/include/linux/percpu_counter.h	2011-04-14 09:09:14.000000000 +0800
> > @@ -89,9 +89,20 @@ struct percpu_counter {
> >  	s64 count;
> >  };
> >  
> > -static inline int percpu_counter_init(struct percpu_counter *fbc, s64 amount)
> > +static inline void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
> >  {
> > +#if BITS_PER_LONG == 32
> > +	preempt_disable();
> >  	fbc->count = amount;
> > +	preempt_enable();
> > +#else
> > +	fbc->count = amount;
> > +#endif
> > +}
> > +
> > +static inline int percpu_counter_init(struct percpu_counter *fbc, s64 amount)
> > +{
> > +	percpu_counter_set(fbc, amount);
> >  	return 0;
> >  }
> >  
> 
> Please dont do this and obfuscate the code.
> 
> percpu_counter_init() cannot be racy.
> 
> Its like saying spin_lock_init() could be racy.
> 
> No other thread/cpu is supposed to use the object if not yet
> initialized.
that's true. I just want the code looks consistent and this doesn't
really obfuscate the code to me.

Thanks,
Shaohua

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