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! This bug will cause redundant rcu_barrier_sched() called.
not too harmful.

----- corresponding:

free_old_closure() side:           change ptype side:

load struct marker_entry.tmp1   |
--------------------------------|--------------------------------
                                |  load struct marker_entry.tmp1
change rcu_pending bit in tmp1  |
                                |  change ptype bit in tmp1
                                |  store tmp1
--------------------------------|--------------------------------
store tmp1                      |

now this result equals that change ptype side do not change ptype
bit, bug! this bug cause marker_probe_cb() access to invalid memory.
oops!

see also: http://en.wikipedia.org/wiki/Bit_field

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/marker.c b/kernel/marker.c
index 3b75d0e8b5a4ede50def07c88bdaad5fd7089415..e9c6b2bc9400627cf183382ee55933333f0ee83b 100644 (file)
--- 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;
+       int rcu_pending;
        unsigned char ptype:1;
        char name[0];   /* Contains name'\0'format'\0' */
 };