Re: IORING_OP_READ and O_NONBLOCK behaviour

[Jens Axboe <axboe@kernel.dk>]

On 9/2/20 9:26 AM, Pavel Begunkov wrote:
> On 02/09/2020 17:45, Jens Axboe wrote:
>> On 9/2/20 4:09 AM, Norman Maurer wrote:
>>> Hi there,
>>>
>>> We are currently working on integrating io_uring into netty and found
>>> some “suprising” behaviour which seems like a bug to me.
>>>
>>> When a socket is marked as non blocking (accepted with O_NONBLOCK flag
>>> set) and there is no data to be read IORING_OP_READ should complete
>>> directly with EAGAIN or EWOULDBLOCK. This is not the case and it
>>> basically blocks forever until there is some data to read. Is this
>>> expected ?
>>>
>>> This seems to be somehow related to a bug that was fixed for
>>> IO_URING_ACCEPT with non blocking sockets:
>>>
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.8&id=e697deed834de15d2322d0619d51893022c90ea2
>>
>> I agree with you that this is a bug, in general it's useful (and
>> expected) that we'd return -EAGAIN for that case. I'll take a look.
>>
> 
> That's I mentioned that doing retries for nonblock requests in
> io_wq_submit_work() doesn't look consistent. I think killing it
> off may help.

Right, we should not retry those _in general_, the exception is regular
files or block devices to handle IOPOLL retry where we do need it. The
below is what I came up with for this one. Might not hurt to make this
more explicit for 5.10.


commit c78e0f02c3861b5b176b2f79552677b3604deb76
Author: Jens Axboe <axboe@kernel.dk>
Date:   Wed Sep 2 09:30:31 2020 -0600

    io_uring: no read-retry on -EAGAIN error and O_NONBLOCK marked file
    
    Actually two things that need fixing up here:
    
    - The io_rw_reissue() -EAGAIN retry is explicit to block devices and
      regular files, so don't ever attempt to do that on other types of
      files.
    
    - If we hit -EAGAIN on a nonblock marked file, don't arm poll handler for
      it. It should just complete with -EAGAIN.
    
    Cc: stable@vger.kernel.org
    Reported-by: Norman Maurer <norman.maurer@googlemail.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index b1ccd7072d93..dc27cd5b8ad6 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2300,8 +2300,11 @@ static bool io_resubmit_prep(struct io_kiocb *req, int error)
 static bool io_rw_reissue(struct io_kiocb *req, long res)
 {
 #ifdef CONFIG_BLOCK
+	umode_t mode = file_inode(req->file)->i_mode;
 	int ret;
 
+	if (!S_ISBLK(mode) && !S_ISREG(mode))
+		return false;
 	if ((res != -EAGAIN && res != -EOPNOTSUPP) || io_wq_current_is_worker())
 		return false;
 
@@ -3146,6 +3149,9 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 		/* IOPOLL retry should happen for io-wq threads */
 		if (!force_nonblock && !(req->ctx->flags & IORING_SETUP_IOPOLL))
 			goto done;
+		/* no retry on NONBLOCK marked file */
+		if (kiocb->ki_flags & IOCB_NOWAIT)
+			goto done;
 		/* some cases will consume bytes even on error returns */
 		iov_iter_revert(iter, iov_count - iov_iter_count(iter));
 		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);

-- 
Jens Axboe