Re: [PATCH V2] rdma_rxe: call comp_handler without holding cq->cq_lock

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Aug 22, 2025 at 10:19:41AM +0200, Philipp Reisner wrote:
> Allow the comp_handler callback implementation to call ib_poll_cq().
> A call to ib_poll_cq() calls rxe_poll_cq() with the rdma_rxe driver.
> And rxe_poll_cq() locks cq->cq_lock. That leads to a spinlock deadlock.
> 
> The Mellanox and Intel drivers allow a comp_handler callback
> implementation to call ib_poll_cq().
> 
> Avoid the deadlock by calling the comp_handler callback without
> holding cq->cq_lock.

I spent some time looking at this, and I think the basic statement
above is right. The comp_handler should be able to call poll_cq/etc

rxe holding a lock it used to push a CQE is not correct.

However! The comp_handler is also supposed to be single threaded by
the driver, I don't think ULPs are prepared to handle concurrent calls
to comp_handler.

Other HW drivers run their comp_handlers from an EQ which is both
single threaded and does not exclude poll_cq/etc.

So while removing the cq lock here is correct from the perspective of
allowing poll_cq, I could not find any locking in rxe that made
do_complete() be single threaded.

Please send a v2, either explain how the do_complete is single
threaded in a comment above the comp_handler call, or make it be
single threaded.

Thanks,
Jason