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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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