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authorPavel Begunkov <asml.silence@gmail.com>2022-11-11 16:51:29 +0000
committerJens Axboe <axboe@kernel.dk>2022-11-11 09:59:27 -0700
commit30a33669fa21cd3dc7d92a00ba736358059014b7 (patch)
tree6cd63d9678ff7392667e8d2fa3df741dc7cecebf /io_uring
parent3851d25c75ed03117268a8feb34adca5a843a126 (diff)
io_uring/poll: fix double poll req->flags races
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@gmail.com> Link: https://lore.kernel.org/r/b7fab2d502f6121a7d7b199fe4d914a43ca9cdfd.1668184658.git.asml.silence@gmail.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'io_uring')
-rw-r--r--io_uring/poll.c29
1 files 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 */