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Subject[PATCH 4.9 20/54] epoll: EPOLL_CTL_ADD: close the race in decision to take fast path
Date
From: Al Viro <viro@zeniv.linux.org.uk>

commit fe0a916c1eae8e17e86c3753d13919177d63ed7e upstream.

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@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/eventpoll.c | 1 +
1 file changed, 1 insertion(+)

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1926,6 +1926,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in
mutex_lock_nested(&ep->mtx, 0);
if (op == EPOLL_CTL_ADD) {
if (!list_empty(&f.file->f_ep_links) ||
+ ep->gen == loop_check_gen ||
is_file_epoll(tf.file)) {
full_check = 1;
mutex_unlock(&ep->mtx);

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