Messages in this thread Patch in this message | | | Subject | [PATCH 1/1] rculist: Replaced list_first_entry_rcu() with list_first_or_null_rcu() | From | Michel Machado <> | Date | Tue, 10 Apr 2012 14:07:40 -0400 |
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Replaced list_first_entry_rcu() with list_first_or_null_rcu() because list_first_entry_rcu() is not safe as one can find in the comment that this patch also adds.
This patch incorporated Paul's suggestions to 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 external code that uses it is probably broken already.
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> ---
diff --git a/include/linux/rculist.h b/include/linux/rculist.h index d079290..ef5da30 100644 --- a/include/linux/rculist.h +++ b/include/linux/rculist.h @@ -228,18 +228,43 @@ 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
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