linux-kernel - Re: [PATCH] markers: bit-field is not thread-safe nor smp-safe

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Date:	Fri, 10 Oct 2008 13:26:36 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	kosaki.motohiro@...fujitsu.com, Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] markers: bit-field is not thread-safe nor smp-safe

Hi Lai-san,

> 
> bit-field is not thread-safe nor smp-safe.
> 
> struct marker_entry.rcu_pending is not protected by any lock
> in rcu-callback free_old_closure().
> so we must turn it into a safe type.

hmmm
however, char also doesn't smp-safe because some architecture doesn't have
any byte load/store instruction.

It seems bogus solution to me ;)


> detail:
> 
> I suppose rcu_pending and ptype are store in struct marker_entry.tmp1
> 
> free_old_closure() side:           change ptype side:
> 
>                                 |  load struct marker_entry.tmp1
> --------------------------------|--------------------------------
>                                 |  change ptype bit in tmp1
> load struct marker_entry.tmp1   |
> change rcu_pending bit in tmp1  |
> store tmp1                      |
> --------------------------------|--------------------------------
>                                 |  store tmp1
> 
> now this result equals that free_old_closure() do not change rcu_pending bit, bug.
> 
> see also: http://en.wikipedia.org/wiki/Bit_field
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/kernel/marker.c b/kernel/marker.c
> index 7d1faec..4777218 100644
> --- a/kernel/marker.c
> +++ b/kernel/marker.c
> @@ -62,7 +62,7 @@ struct marker_entry {
>  	int refcount;	/* Number of times armed. 0 if disarmed. */
>  	struct rcu_head rcu;
>  	void *oldptr;
> -	unsigned char rcu_pending:1;
> +	unsigned char rcu_pending;
>  	unsigned char ptype:1;
>  	char name[0];	/* Contains name'\0'format'\0' */
>  };



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