Re: [PATCH for-rc v1] RDMA/rxe: Avoid CQ polling hang triggered by CQ resize

[Jason Gunthorpe <jgg@nvidia.com>]

On Sun, Aug 17, 2025 at 12:37:52PM +0000, Daisuke Matsuda wrote:
> When running the test_resize_cq testcase from rdma-core, polling a
> completion queue from userspace may occasionally hang and eventually fail
> with a timeout:
> =====
> ERROR: test_resize_cq (tests.test_cq.CQTest.test_resize_cq)
> Test resize CQ, start with specific value and then increase and decrease
> ----------------------------------------------------------------------
> Traceback (most recent call last):
>     File "/root/deb/rdma-core/tests/test_cq.py", line 135, in test_resize_cq
>       u.poll_cq(self.client.cq)
>     File "/root/deb/rdma-core/tests/utils.py", line 687, in poll_cq
>       wcs = _poll_cq(cq, count, data)
>             ^^^^^^^^^^^^^^^^^^^^^^^^^
>     File "/root/deb/rdma-core/tests/utils.py", line 669, in _poll_cq
>       raise PyverbsError(f'Got timeout on polling ({count} CQEs remaining)')
> pyverbs.pyverbs_error.PyverbsError: Got timeout on polling (1 CQEs
> remaining)
> =====
> 
> The issue is caused when rxe_cq_post() fails to post a CQE due to the queue
> being temporarily full, and the CQE is effectively lost. To mitigate this,
> add a bounded busy-wait with fallback rescheduling so that CQE does not get
> lost.

Nothing should spin like this, that is not right.

The CQE queue is intended to be properly sized for the workload and I
seem to remember overflowing a CQE can just ERR the QP.

Alternatively you can drop the packet and do nothing.

But not spin hoping something emptys it.

Jason