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

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Date:	Tue, 12 Apr 2011 11:03:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, cl@...ux.com,
	tj@...nel.org
Subject: Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

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.

This interface assumes caller take the appropriate locking.



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