Messages in this thread Patch in this message | | | Subject | [PATCH 1/1] rculist: Made list_first_entry_rcu usable | From | Michel Machado <> | Date | Mon, 02 Apr 2012 21:42:34 -0400 |
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The macro list_first_entry_rcu assumed that the passed list is not empty as its counterpart list_first_entry does. However, one can test that a list is not empty with list_empty before calling list_first_entry, whereas neither exists list_empty_rcu, nor is advisable to add it as the example below shows.
Assuming that list_empty_rcu is available, one could write the following snippet:
if (!list_empty_rcu(mylist)) { struct foo *bar = list_first_entry_rcu(mylist, struct foo, list_member); do_something(bar); }
The problem with this snippet is the following racing condition: 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.
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: Dipankar Sarma <dipankar@in.ibm.com> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> --- Please CC my e-mail address while replying this message because I don't subscribe this mailing list due to its high volume; thanks.
diff --git a/include/linux/rculist.h b/include/linux/rculist.h index d079290..866d3ec 100644 --- a/include/linux/rculist.h +++ b/include/linux/rculist.h @@ -233,13 +233,16 @@ static inline void list_splice_init_rcu(struct list_head *list, * @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) + ({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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