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authorMichel Machado <michel@digirati.com.br>2012-04-10 14:07:40 -0400
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-04-24 20:54:49 -0700
commitf88022a4f650ac1778cafcc17d2e522283bdf590 (patch)
tree650a2185c949330002bb4dce090f3257f2779ee6 /include/linux/rculist.h
parent559f9badd11ddf399f88b18b4c0f110fd511ae53 (diff)
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rcu: Replace list_first_entry_rcu() with list_first_or_null_rcu()
The list_first_entry_rcu() macro is inherently unsafe because it cannot be applied to an empty list. But because RCU readers do not exclude updaters, a list might become empty between the time that list_empty() claimed it was non-empty and the time that list_first_entry_rcu() is invoked. Therefore, the list_empty() test cannot be separated from the list_first_entry_rcu() call. This commit therefore combines these to macros to create a new list_first_or_null_rcu() macro that replaces the old (and unsafe) list_first_entry_rcu() macro. This patch incorporates Paul's review comments on the previous version of this patch available here: https://lkml.org/lkml/2012/4/2/536 This patch cannot break any upstream code because list_first_entry_rcu() is not being used anywhere in the kernel (tested with grep(1)), and any external code using it is probably broken as a result of using it. Signed-off-by: Michel Machado <michel@digirati.com.br> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> CC: Dipankar Sarma <dipankar@in.ibm.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'include/linux/rculist.h')
-rw-r--r--include/linux/rculist.h33
1 files changed, 29 insertions, 4 deletions
diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index a20c05096231..e0f0fab20415 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -233,18 +233,43 @@ static inline void list_splice_init_rcu(struct list_head *list,
})
/**
- * list_first_entry_rcu - get the first element from a list
+ * Where are list_empty_rcu() and list_first_entry_rcu()?
+ *
+ * Implementing those functions following their counterparts list_empty() and
+ * list_first_entry() is not advisable because they lead to subtle race
+ * conditions as the following snippet shows:
+ *
+ * if (!list_empty_rcu(mylist)) {
+ * struct foo *bar = list_first_entry_rcu(mylist, struct foo, list_member);
+ * do_something(bar);
+ * }
+ *
+ * The list may not be empty when list_empty_rcu checks it, but it may be when
+ * list_first_entry_rcu rereads the ->next pointer.
+ *
+ * Rereading the ->next pointer is not a problem for list_empty() and
+ * list_first_entry() because they would be protected by a lock that blocks
+ * writers.
+ *
+ * See list_first_or_null_rcu for an alternative.
+ */
+
+/**
+ * list_first_or_null_rcu - get the first element from a list
* @ptr: the list head to take the element from.
* @type: the type of the struct this is embedded in.
* @member: the name of the list_struct within the struct.
*
- * Note, that list is expected to be not empty.
+ * Note that if the list is empty, it returns NULL.
*
* This primitive may safely run concurrently with the _rcu list-mutation
* primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
*/
-#define list_first_entry_rcu(ptr, type, member) \
- list_entry_rcu((ptr)->next, type, member)
+#define list_first_or_null_rcu(ptr, type, member) \
+ ({struct list_head *__ptr = (ptr); \
+ struct list_head __rcu *__next = list_next_rcu(__ptr); \
+ likely(__ptr != __next) ? container_of(__next, type, member) : NULL; \
+ })
/**
* list_for_each_entry_rcu - iterate over rcu list of given type