linux-kernel - Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

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Date:	Wed, 13 Apr 2011 09:01:05 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>, "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

On Tue, 2011-04-12 at 17:03 +0800, Eric Dumazet wrote:
> Le mardi 12 avril 2011 à 16:04 +0800, Shaohua Li a écrit :
> > 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 |   43 +++++++++++++++++++++++++++++++----------
> >  1 file changed, 33 insertions(+), 10 deletions(-)
> > 
> > Index: linux/include/linux/percpu_counter.h
> > ===================================================================
> > --- linux.orig/include/linux/percpu_counter.h	2011-04-12 15:48:44.000000000 +0800
> > +++ linux/include/linux/percpu_counter.h	2011-04-12 15:48:54.000000000 +0800
> > @@ -54,7 +54,15 @@ static inline s64 percpu_counter_sum(str
> >  
> >  static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> >  {
> > +#if BITS_PER_LONG == 32
> > +	s64 count;
> > +	spin_lock(&fbc->lock);
> > +	count = fbc->count;
> > +	spin_unlock(&fbc->lock);
> > +	return count;
> > +#else
> >  	return fbc->count;
> > +#endif
> >  }
> >  
> 
> Hmm... did you test this with LOCKDEP on ?
> 
> You add a possible deadlock here.
> 
> Hint : Some percpu_counter are used from irq context.
there are some places we didn't disable interrupt, for example
percpu_counter_add. So the API isn't irq safe to me.

> This interface assumes caller take the appropriate locking.
no comments say this, and some places we don't hold locking.
for example, meminfo_proc_show.

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