epoll: annotate racy check
Epoll relies on a racy fastpath check during __fput() in
eventpoll_release() to avoid the hit of pointlessly acquiring a
semaphore. Annotate that race by using WRITE_ONCE() and READ_ONCE().
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/20240925-fungieren-anbauen-79b334b00542@brauner
Reviewed-by: Jan Kara <[email protected]>
Reported-by: [email protected]
Signed-off-by: Christian Brauner <[email protected]>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 1ae4542..90fbab6 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -823,7 +823,8 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
to_free = NULL;
head = file->f_ep;
if (head->first == &epi->fllink && !epi->fllink.next) {
- file->f_ep = NULL;
+ /* See eventpoll_release() for details. */
+ WRITE_ONCE(file->f_ep, NULL);
if (!is_file_epoll(file)) {
struct epitems_head *v;
v = container_of(head, struct epitems_head, epitems);
@@ -1603,7 +1604,8 @@ static int attach_epitem(struct file *file, struct epitem *epi)
spin_unlock(&file->f_lock);
goto allocate;
}
- file->f_ep = head;
+ /* See eventpoll_release() for details. */
+ WRITE_ONCE(file->f_ep, head);
to_free = NULL;
}
hlist_add_head_rcu(&epi->fllink, file->f_ep);
