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io_uring/poll: fix double poll req->flags races

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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: 49f1c68e04 ("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>
This commit is contained in:
Pavel Begunkov 2022-11-11 16:51:29 +00:00 committed by Jens Axboe
parent 3851d25c75
commit 30a33669fa
1 changed files with 17 additions and 12 deletions
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29
io_uring/poll.c
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@ -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 */