From: Pavel Begunkov <asml.silence@xxxxxxxxx>
[ Upstream commit 30a33669fa21cd3dc7d92a00ba736358059014b7 ]
io_poll_double_prepare() | io_poll_wake()
| poll->head = NULL
smp_load(&poll->head); /* NULL */ |
flags = req->flags; |
| req->flags &= ~SINGLE_POLL;
req->flags = flags | DOUBLE_POLL |
The idea behind io_poll_double_prepare() is to serialise with the
first poll entry by taking the wq lock. However, it's not safe to assume
that io_poll_wake() is not running when we can't grab the lock and so we
may race modifying req->flags.
Skip double poll setup if that happens. It's ok because the first poll
entry will only be removed when it's definitely completing, e.g.
pollfree or oneshot with a valid mask.
Fixes: 49f1c68e048f1 ("io_uring: optimise submission side poll_refs")
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
Link: https://lore.kernel.org/r/b7fab2d502f6121a7d7b199fe4d914a43ca9cdfd.1668184658.git.asml.silence@xxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
io_uring/poll.c | 29 +++++++++++++++++------------
1 file changed, 17 insertions(+), 12 deletions(-)
diff --git a/io_uring/poll.c b/io_uring/poll.c
index 0d9f49c575e0..97c214aa688c 100644
--- a/io_uring/poll.c
+++ b/io_uring/poll.c
@@ -394,7 +394,8 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
return 1;
}
-static void io_poll_double_prepare(struct io_kiocb *req)
+/* fails only when polling is already completing by the first entry */
+static bool io_poll_double_prepare(struct io_kiocb *req)
{
struct wait_queue_head *head;
struct io_poll *poll = io_poll_get_single(req);
@@ -403,20 +404,20 @@ static void io_poll_double_prepare(struct io_kiocb *req)
rcu_read_lock();
head = smp_load_acquire(&poll->head);
/*
- * poll arm may not hold ownership and so race with
- * io_poll_wake() by modifying req->flags. There is only one
- * poll entry queued, serialise with it by taking its head lock.
+ * poll arm might not hold ownership and so race for req->flags with
+ * io_poll_wake(). There is only one poll entry queued, serialise with
+ * it by taking its head lock. As we're still arming the tw hanlder
+ * is not going to be run, so there are no races with it.
*/
- if (head)
+ if (head) {
spin_lock_irq(&head->lock);
-
- req->flags |= REQ_F_DOUBLE_POLL;
- if (req->opcode == IORING_OP_POLL_ADD)
- req->flags |= REQ_F_ASYNC_DATA;
-
- if (head)
+ req->flags |= REQ_F_DOUBLE_POLL;
+ if (req->opcode == IORING_OP_POLL_ADD)
+ req->flags |= REQ_F_ASYNC_DATA;
spin_unlock_irq(&head->lock);
+ }
rcu_read_unlock();
+ return !!head;
}
static void __io_queue_proc(struct io_poll *poll, struct io_poll_table *pt,
@@ -454,7 +455,11 @@ static void __io_queue_proc(struct io_poll *poll, struct io_poll_table *pt,
/* mark as double wq entry */
wqe_private |= IO_WQE_F_DOUBLE;
io_init_poll_iocb(poll, first->events, first->wait.func);
- io_poll_double_prepare(req);
+ if (!io_poll_double_prepare(req)) {
+ /* the request is completing, just back off */
+ kfree(poll);
+ return;
+ }
*poll_ptr = poll;
} else {
/* fine to modify, there is no poll queued to race with us */
--
2.35.1
