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author | Michel Machado <michel@digirati.com.br> | 2012-04-10 22:07:40 +0400 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2012-04-25 07:54:49 +0400 |
commit | f88022a4f650ac1778cafcc17d2e522283bdf590 (patch) | |
tree | 650a2185c949330002bb4dce090f3257f2779ee6 | |
parent | 559f9badd11ddf399f88b18b4c0f110fd511ae53 (diff) | |
download | linux-f88022a4f650ac1778cafcc17d2e522283bdf590.tar.xz |
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>
-rw-r--r-- | include/linux/rculist.h | 33 |
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 |