[RFC][PATCH 2/8] epoll: remove ep from visited_list when freeing ep

[Hou Tao]

Before the removal of epmutex, the acquisition of epmutex in
ep_free() will prevent the freeing of ep, so it's OK to access
ep in visited_list in ep_loop_check().

To ensure the validity of ep when clearing visited_list, we need
to remove ep from visited_list when freeing ep. If the ep had been
added to the visited_list, we need to wait for its removal.

Signed-off-by: Hou Tao <hout...@huawei.com>
---
 fs/eventpoll.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 26ab0c5..44ea587 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -862,6 +862,18 @@ static void ep_free(struct eventpoll *ep)
        }
        mutex_unlock(&ep->mtx);
 
+       /*
+        * ep will not been added to visited_list, because ep_ctrl()
+        * can not get its reference and can not reference it by the
+        * corresponding epitem. The only possible operation is list_del_init,
+        * so it's OK to use list_empty_careful() here.
+        */
+       if (!list_empty_careful(&ep->visited_list_link)) {
+               mutex_lock(&epmutex);
+               list_del_init(&ep->visited_list_link);
+               mutex_unlock(&epmutex);
+       }
+
        mutex_destroy(&ep->mtx);
        free_uid(ep->user);
        wakeup_source_unregister(ep->ws);
@@ -1039,6 +1051,7 @@ static int ep_alloc(struct eventpoll **pep)
        ep->rbr = RB_ROOT_CACHED;
        ep->ovflist = EP_UNACTIVE_PTR;
        ep->user = user;
+       INIT_LIST_HEAD(&ep->visited_list_link);
 
        *pep = ep;
 
@@ -1928,7 +1941,7 @@ static int ep_loop_check(struct eventpoll *ep, struct 
file *file)
        list_for_each_entry_safe(ep_cur, ep_next, &visited_list,
                                                        visited_list_link) {
                ep_cur->visited = 0;
-               list_del(&ep_cur->visited_list_link);
+               list_del_init(&ep_cur->visited_list_link);
        }
        return ret;
 }
-- 
2.7.5