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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Sep 09, 2025 at 04:48:19PM +0200, Philipp Reisner wrote:
> On Mon, Sep 8, 2025 at 4:25 PM Leon Romanovsky <leon@kernel.org> 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.

Is this even legal in verbs? I'm not sure you can do pull cq from a
interrupt driven comp handler.. Is something already doing this intree?

Jason