Re: [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Ming Lei <ming.lei@redhat.com>]

On Tue, Apr 05, 2022 at 08:20:24PM -0600, Jens Axboe wrote:
> On 4/3/22 5:45 AM, Ming Lei wrote:
> > -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is
> > set in io_complete_rw_iopoll(), but the req never gets chance to be handled.
> > io_iopoll_check doesn't handle this situation, and io hang can be caused.
> > 
> > Current dm io polling may return -EAGAIN after bio submission is
> > returned, also blk-throttle might trigger this situation too.
> 
> I don't think this is necessarily safe. Handling REQ_F_ISSUE from within
> the issue path is fine, as the request hasn't been submitted yet and
> hence we know that passed in structs are still stable. Once you hit it
> when polling for it, the io_uring_enter() call to submit requests has
> potentially already returned, and now we're in a second call where we
> are polling for requests. If we're doing eg an IORING_OP_READV, the
> original iovec may no longer be valid and we cannot safely re-import
> data associated with it.

Yeah, this reissue is really not safe, thanks for the input.

I guess the only way is to complete the cqe for this situation.

> 
> Hence I don't think the patch is safe and we cannot reliably handle this
> scenario. dm would need to retry internally for this.

Another scenario might be bio throttle, which may delay submit_bio into kthrotld
wq context, where request allocation can return -EAGAIN too.

But I don't reproduce the problem in this scenario yet.


Thanks,
Ming