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

Jens Axboe
2020-02-12T20:25:22Z

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) {
+ 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);
+ 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)
{
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);
+ 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);
--
2.25.0