io_uring: ensure we clear io_kiocb->result before each issue

commit 6873e0bd6a9cb14ecfadd89d9ed9698ff1761902 upstream.

We use io_kiocb->result == -EAGAIN as a way to know if we need to
re-submit a polled request, as -EAGAIN reporting happens out-of-line
for IO submission failures. This field is cleared when we originally
allocate the request, but it isn't reset when we retry the submission
from async context. This can cause issues where we think something
needs a re-issue, but we're really just reading stale data.

Reset ->result whenever we re-prep a request for polled submission.

Cc: stable@vger.kernel.org
Fixes: 9e645e1105ca ("io_uring: add support for sqe links")
Reported-by: NBijan Mottahedeh <bijan.mottahedeh@oracle.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

fs/io_uring.c

@@ -1211,6 +1211,7 @@ static int io_prep_rw(struct io_kiocb *req, const struct sqe_submit *s,
 
 		kiocb->ki_flags |= IOCB_HIPRI;
 		kiocb->ki_complete = io_complete_rw_iopoll;
+		req->result = 0;
 	} else {
 		if (kiocb->ki_flags & IOCB_HIPRI)
 			return -EINVAL;