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

[Philipp Reisner <philipp.reisner@linbit.com>]

On Tue, Sep 9, 2025 at 5:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> 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?
>

The file drivers/infiniband/sw/rdmavt/cq.c has this comment:
/*
* The completion handler will most likely rearm the notification
* and poll for all pending entries.  If a new completion entry
* is added while we are in this routine, queue_work()
* won't call us again until we return so we check triggered to
* see if we need to call the handler again.
*/

Also, Intel and Mellanox cards and drivers allow calling ib_poll_cq()
from the completion handler.

The problem exists only with the RXE driver.