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

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Message-ID: <20250908142457.GA341237@unreal>
Date: Mon, 8 Sep 2025 17:24:57 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Philipp Reisner <philipp.reisner@...bit.com>
Cc: Zhu Yanjun <yanjun.zhu@...ux.dev>, Jason Gunthorpe <jgg@...pe.ca>,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] rdma_rxe: call comp_handler without holding
 cq->cq_lock

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.

Can you please be more specific about the deadlock?
Please write call stack to describe it.

> 
> 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.
> 
> Changelog:
> v1: https://lore.kernel.org/all/20250806123921.633410-1-philipp.reisner@linbit.com/
> v1 -> v2:
> - Only reset cq->notify to 0 when invoking the comp_handler
> ====================
> 
> Signed-off-by: Philipp Reisner <philipp.reisner@...bit.com>
> Reviewed-by: Zhu Yanjun <yanjun.zhu@...ux.dev>
> ---
>  drivers/infiniband/sw/rxe/rxe_cq.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/infiniband/sw/rxe/rxe_cq.c b/drivers/infiniband/sw/rxe/rxe_cq.c
> index fffd144d509e..95652001665d 100644
> --- a/drivers/infiniband/sw/rxe/rxe_cq.c
> +++ b/drivers/infiniband/sw/rxe/rxe_cq.c
> @@ -88,6 +88,7 @@ int rxe_cq_post(struct rxe_cq *cq, struct rxe_cqe *cqe, int solicited)
>  	int full;
>  	void *addr;
>  	unsigned long flags;
> +	bool invoke_handler = false;
>  
>  	spin_lock_irqsave(&cq->cq_lock, flags);
>  
> @@ -113,11 +114,14 @@ int rxe_cq_post(struct rxe_cq *cq, struct rxe_cqe *cqe, int solicited)
>  	if ((cq->notify & IB_CQ_NEXT_COMP) ||
>  	    (cq->notify & IB_CQ_SOLICITED && solicited)) {
>  		cq->notify = 0;
> -		cq->ibcq.comp_handler(&cq->ibcq, cq->ibcq.cq_context);
> +		invoke_handler = true;
>  	}
>  
>  	spin_unlock_irqrestore(&cq->cq_lock, flags);
>  
> +	if (invoke_handler)
> +		cq->ibcq.comp_handler(&cq->ibcq, cq->ibcq.cq_context);
> +
>  	return 0;
>  }
>  
> -- 
> 2.50.1