Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task work

On Aug 10, 2021, at 2:32 PM, Pavel Begunkov [off-list ref] wrote:

On 8/10/21 9:28 AM, Nadav Amit wrote:

quoted

Unfortunately, there seems to be yet another issue (unless my code
somehow caused it). It seems that when SQPOLL is used, there are cases
in which we get stuck in io_uring_cancel_sqpoll() when tctx_inflight()
never goes down to zero.

Debugging... (while also trying to make some progress with my code)

It's most likely because a request has been lost (mis-refcounted).
Let us know if you need any help. Would be great to solve it for 5.14.
quick tips: 

1) if not already, try out Jens' 5.14 branch
git://git.kernel.dk/linux-block io_uring-5.14

2) try to characterise the io_uring use pattern. Poll requests?
Read/write requests? Send/recv? Filesystem vs bdev vs sockets?

If easily reproducible, you can match io_alloc_req() with it
getting into io_dismantle_req();

So actually the problem is more of a missing IO-uring functionality that I need. When an I/O is queued for async completion (i.e., after returning -EIOCBQUEUED), there should be a way for io-uring to cancel these I/Os if needed. Otherwise they might potentially never complete, as happens in my use-case.

AIO has ki_cancel() for this matter. So I presume the proper solution would be to move ki_cancel() from aio_kiocb to kiocb so it can be used by both io-uring and aio. And then - to use this infrastructure.

But it is messy. There is already a bug in the (few) uses of kiocb_set_cancel_fn() that blindly assume AIO is used and not IO-uring. Then, I am not sure about some things in the AIO code. Oh boy. I’ll work on an RFC.