From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[ Upstream commit fe0a916c1eae8e17e86c3753d13919177d63ed7e ]
Checking for the lack of epitems refering to the epoll we want to insert into
is not enough; we might have an insertion of that epoll into another one that
has already collected the set of files to recheck for excessive reverse paths,
but hasn't gotten to creating/inserting the epitem for it.
However, any such insertion in progress can be detected - it will update the
generation count in our epoll when it's done looking through it for files
to check. That gets done under ->mtx of our epoll and that allows us to
detect that safely.
We are *not* holding epmutex here, so the generation count is not stable.
However, since both the update of ep->gen by loop check and (later)
insertion into ->f_ep_link are done with ep->mtx held, we are fine -
the sequence is
grab epmutex
bump loop_check_gen
...
grab tep->mtx // 1
tep->gen = loop_check_gen
...
drop tep->mtx // 2
...
grab tep->mtx // 3
...
insert into ->f_ep_link
...
drop tep->mtx // 4
bump loop_check_gen
drop epmutex
and if the fastpath check in another thread happens for that
eventpoll, it can come
* before (1) - in that case fastpath is just fine
* after (4) - we'll see non-empty ->f_ep_link, slow path
taken
* between (2) and (3) - loop_check_gen is stable,
with ->mtx providing barriers and we end up taking slow path.
Note that ->f_ep_link emptiness check is slightly racy - we are protected
against insertions into that list, but removals can happen right under us.
Not a problem - in the worst case we'll end up taking a slow path for
no good reason.
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
fs/eventpoll.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 82ab9a25f12f2..16313180e4c16 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2181,6 +2181,7 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
goto error_tgt_fput;
if (op == EPOLL_CTL_ADD) {
if (!list_empty(&f.file->f_ep_links) ||
+ ep->gen == loop_check_gen ||
is_file_epoll(tf.file)) {
mutex_unlock(&ep->mtx);
error = epoll_mutex_lock(&epmutex, 0, nonblock);
--
2.25.1
