Re: [PATCH 3/3] io_uring: allow POLL_ADD with double poll_wait() users

Pavel Begunkov
2020-02-13T15:50:18Z

Hi,

There is a couple of comments below

On 2/12/2020 11:25 PM, Jens Axboe wrote:
> Some file descriptors use separate waitqueues for their f_ops->poll()
> handler, most commonly one for read and one for write. The io_uring
> poll implementation doesn't work with that, as the 2nd poll_wait()
> call will cause the io_uring poll request to -EINVAL.
>
> This is particularly a problem now that pipes were switched to using
> multiple wait queues (commit 0ddad21d3e99), but it also affects tty
> devices and /dev/random as well. This is a big problem for event loops
> where some file descriptors work, and others don't.
>
> With this fix, io_uring handles multiple waitqueues.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
> fs/io_uring.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 74 insertions(+), 5 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 9cd2ce3b8ad9..9f00f30e1790 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -3440,10 +3440,27 @@ static int io_connect(struct io_kiocb *req, struct io_kiocb **nxt,
> #endif
> }
>
> +static void io_poll_remove_double(struct io_kiocb *req)
> +{
> + struct io_poll_iocb *poll = (struct io_poll_iocb *) req->io;
> +
> + if (poll && poll->head) {
> + unsigned long flags;
> +
> + spin_lock_irqsave(&poll->head->lock, flags);
> + list_del_init(&poll->wait.entry);
> + if (poll->wait.private)
> + refcount_dec(&req->refs);
> + spin_unlock_irqrestore(&poll->head->lock, flags);
> + }
> +}
> +
> static void io_poll_remove_one(struct io_kiocb *req)
> {
> struct io_poll_iocb *poll = &req->poll;
>
> + io_poll_remove_double(req);
> +
> spin_lock(&poll->head->lock);
> WRITE_ONCE(poll->canceled, true);
> if (!list_empty(&poll->wait.entry)) {
> @@ -3679,10 +3696,38 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
> if (mask && !(mask & poll->events))
> return 0;
>
> + io_poll_remove_double(req);
> __io_poll_wake(req, &req->poll, mask);
> return 1;
> }
>
> +static int io_poll_double_wake(struct wait_queue_entry *wait, unsigned mode,
> + int sync, void *key)
> +{
> + struct io_kiocb *req = wait->private;
> + struct io_poll_iocb *poll = (void *) req->io;
> + __poll_t mask = key_to_poll(key);
> + bool done = true;
> +
> + /* for instances that support it check for an event match first: */
> + if (mask && !(mask & poll->events))
> + return 0;
> +
> + if (req->poll.head) {

Can there be concurrent problems?

1. io_poll_wake() -> io_poll_remove_double() is working
awhile the second io_poll_queue_proc() is called.
Then there will be a race for req->io

2. concurrent io_poll_wake() and io_poll_double_wake()

> + unsigned long flags;
> +
> + spin_lock_irqsave(&req->poll.head->lock, flags);
> + done = list_empty(&req->poll.wait.entry);
> + if (!done)
> + list_del_init(&req->poll.wait.entry);
> + spin_unlock_irqrestore(&req->poll.head->lock, flags);
> + }
> + if (!done)
> + __io_poll_wake(req, poll, mask);

It's always false if we didn't hit the block under `req->poll.head`, so
it may be placed there along with @done declaration.

> + refcount_dec(&req->refs);
> + return 1;
> +}
> +
> struct io_poll_table {
> struct poll_table_struct pt;
> struct io_kiocb *req;
> @@ -3693,15 +3738,38 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head,
> struct poll_table_struct *p)
> {

May this be called concurrently? (at least in theory)

> struct io_poll_table *pt = container_of(p, struct io_poll_table, pt);
> + struct io_kiocb *req = pt->req;
> + struct io_poll_iocb *poll = &req->poll;
>
> - if (unlikely(pt->req->poll.head)) {
> - pt->error = -EINVAL;
> - return;
> + /*
> + * If poll->head is already set, it's because the file being polled
> + * use multiple waitqueues for poll handling (eg one for read, one
> + * for write). Setup a separate io_poll_iocb if this happens.
> + */
> + if (unlikely(poll->head)) {
> + /* already have a 2nd entry, fail a third attempt */
> + if (req->io) {
> + pt->error = -EINVAL;
> + return;
> + }
> + poll = kmalloc(sizeof(*poll), GFP_ATOMIC);

Don't see where this is freed

> + if (!poll) {
> + pt->error = -ENOMEM;
> + return;
> + }
> + poll->done = false;
> + poll->canceled = false;
> + poll->events = req->poll.events;
> + INIT_LIST_HEAD(&poll->wait.entry);
> + init_waitqueue_func_entry(&poll->wait, io_poll_double_wake);
> + refcount_inc(&req->refs);
> + poll->wait.private = req;
> + req->io = (void *) poll;
> }
>
> pt->error = 0;
> - pt->req->poll.head = head;
> - add_wait_queue(head, &pt->req->poll.wait);
> + poll->head = head;
> + add_wait_queue(head, &poll->wait);
> }
>
> static void io_poll_req_insert(struct io_kiocb *req)
> @@ -3778,6 +3846,7 @@ static int io_poll_add(struct io_kiocb *req, struct io_kiocb **nxt)
> }
> if (mask) { /* no async, we'd stolen it */
> ipt.error = 0;
> + io_poll_remove_double(req);
> io_poll_complete(req, mask, 0);
> }
> spin_unlock_irq(&ctx->completion_lock);
>

--
Pavel Begunkov