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 04:35:30 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	shaohua.li@...el.com
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	tj@...nel.org, cl@...ux.com
Subject: Re: [patch v3 2/3] percpu_counter: fix code for 32bit systems for
 UP

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.



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