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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2013-03-09 07:38:41 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2013-03-14 13:18:30 -0700
commitf65846a1800ef8c48d1ae1973c30dae4c356a800 (patch)
tree87bd07485309f57d4002a5cd634636e9a2ce3025
parentf6161aa153581da4a3867a2d1a7caf4be19b6ec9 (diff)
list: Fix double fetch of pointer in hlist_entry_safe()
The current version of hlist_entry_safe() fetches the pointer twice, once to test for NULL and the other to compute the offset back to the enclosing structure. This is OK for normal lock-based use because in that case, the pointer cannot change. However, when the pointer is protected by RCU (as in "rcu_dereference(p)"), then the pointer can change at any time. This use case can result in the following sequence of events: 1. CPU 0 invokes hlist_entry_safe(), fetches the RCU-protected pointer as sees that it is non-NULL. 2. CPU 1 invokes hlist_del_rcu(), deleting the entry that CPU 0 just fetched a pointer to. Because this is the last entry in the list, the pointer fetched by CPU 0 is now NULL. 3. CPU 0 refetches the pointer, obtains NULL, and then gets a NULL-pointer crash. This commit therefore applies gcc's "({ })" statement expression to create a temporary variable so that the specified pointer is fetched only once, avoiding the above sequence of events. Please note that it is the caller's responsibility to use rcu_dereference() as needed. This allows RCU-protected uses to work correctly without imposing any additional overhead on the non-RCU case. Many thanks to Eric Dumazet for spotting root cause! Reported-by: CAI Qian <caiqian@redhat.com> Reported-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Li Zefan <lizefan@huawei.com>
-rw-r--r--include/linux/list.h4
1 files changed, 3 insertions, 1 deletions
diff --git a/include/linux/list.h b/include/linux/list.h
index d991cc147c98..6a1f8df9144b 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -667,7 +667,9 @@ static inline void hlist_move_list(struct hlist_head *old,
pos = n)
#define hlist_entry_safe(ptr, type, member) \
- (ptr) ? hlist_entry(ptr, type, member) : NULL
+ ({ typeof(ptr) ____ptr = (ptr); \
+ ____ptr ? hlist_entry(____ptr, type, member) : NULL; \
+ })
/**
* hlist_for_each_entry - iterate over list of given type

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