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

[Philipp Reisner]

On Mon, Sep 8, 2025 at 4:25 PM Leon Romanovsky <leon@xxxxxxxxxx> wrote:
>
> 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.
>
Instead of a call stack, I write it from top to bottom:

The line numbers in the .c files are valid for Linux-6.16:

1  rxe_cq_post()                      [rxe_cq.c:85]
2   spin_lock_irqsave()               [rxe_cq.c:93]
3   cq->ibcq.comp_handler()           [rxe_cq.c:116]
4    some_comp_handler()
5     ib_poll_cq()
6      cq->device->ops.poll_cq()      [ib_verbs.h:4037]
7       rxe_poll_cq()                 [rxe_verbs.c:1165]
8        spin_lock_irqsave()          [rxe_verbs.c:1172]

In line 8 of this call graph, it deadlocks because the spinlock
was already acquired in line 2 of the call graph.

This patch changes that call graph to:
(Line numbers now valid with the patch in discussion applied)

1  rxe_cq_post()                      [rxe_cq.c:85]
2   spin_lock_irqsave()               [rxe_cq.c:93]
3   spin_unlock_irqrestore()          [rxe_cq.c:120]
4   cq->ibcq.comp_handler()           [rxe_cq.c:123]
5    some_comp_handler()
6     ib_poll_cq()
7      cq->device->ops.poll_cq()      [ib_verbs.h:4037]
8       rxe_poll_cq()                 [rxe_verbs.c:1165]
9        spin_lock_irqsave()          [rxe_verbs.c:1172]

With the patch, there is no deadlock in line 9 of the call graph,
as the spinlock was released in line 3.

I hope that helps.

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