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

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Date:	Fri, 10 Oct 2008 11:06:21 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] markers: bit-field is not thread-safe nor smp-safe


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.

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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