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

[Jens Axboe <axboe@kernel.dk>]

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.

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

-- 
Jens Axboe