Re: [PATCH for-next 4/4] RDMA/efa: CQ notifications

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 02, 2021 at 06:17:45PM +0300, Gal Pressman wrote:
> On 02/09/2021 18:10, Jason Gunthorpe wrote:
> > On Thu, Sep 02, 2021 at 06:09:39PM +0300, Gal Pressman wrote:
> >> On 02/09/2021 16:02, Jason Gunthorpe wrote:
> >>> On Thu, Sep 02, 2021 at 10:03:16AM +0300, Gal Pressman wrote:
> >>>> On 01/09/2021 18:36, Jason Gunthorpe wrote:
> >>>>> On Wed, Sep 01, 2021 at 05:24:43PM +0300, Gal Pressman wrote:
> >>>>>> On 01/09/2021 14:57, Jason Gunthorpe wrote:
> >>>>>>> On Wed, Sep 01, 2021 at 02:50:42PM +0300, Gal Pressman wrote:
> >>>>>>>> On 20/08/2021 21:27, Jason Gunthorpe wrote:
> >>>>>>>>> On Wed, Aug 11, 2021 at 06:11:31PM +0300, Gal Pressman wrote:
> >>>>>>>>>> diff --git a/drivers/infiniband/hw/efa/efa_main.c b/drivers/infiniband/hw/efa/efa_main.c
> >>>>>>>>>> index 417dea5f90cf..29db4dec02f0 100644
> >>>>>>>>>> +++ b/drivers/infiniband/hw/efa/efa_main.c
> >>>>>>>>>> @@ -67,6 +67,46 @@ static void efa_release_bars(struct efa_dev *dev, int bars_mask)
> >>>>>>>>>>      pci_release_selected_regions(pdev, release_bars);
> >>>>>>>>>>  }
> >>>>>>>>>>
> >>>>>>>>>> +static void efa_process_comp_eqe(struct efa_dev *dev, struct efa_admin_eqe *eqe)
> >>>>>>>>>> +{
> >>>>>>>>>> +    u16 cqn = eqe->u.comp_event.cqn;
> >>>>>>>>>> +    struct efa_cq *cq;
> >>>>>>>>>> +
> >>>>>>>>>> +    cq = xa_load(&dev->cqs_xa, cqn);
> >>>>>>>>>> +    if (unlikely(!cq)) {
> >>>>>>>>>
> >>>>>>>>> This seems unlikely to be correct, what prevents cq from being
> >>>>>>>>> destroyed concurrently?
> >>>>>>>>>
> >>>>>>>>> A comp_handler cannot be running after cq destroy completes.
> >>>>>>>>
> >>>>>>>> Sorry for the long turnaround, was OOO.
> >>>>>>>>
> >>>>>>>> The CQ cannot be destroyed until all completion events are acked.
> >>>>>>>> https://github.com/linux-rdma/rdma-core/blob/7fd01f0c6799f0ecb99cae03c22cf7ff61ffbf5a/libibverbs/man/ibv_get_cq_event.3#L45
> >>>>>>>> https://github.com/linux-rdma/rdma-core/blob/7fd01f0c6799f0ecb99cae03c22cf7ff61ffbf5a/libibverbs/cmd_cq.c#L208
> >>>>>>>
> >>>>>>> That is something quite different, and in userspace.
> >>>>>>>
> >>>>>>> What in the kernel prevents tha xa_load and the xa_erase from racing together?
> >>>>>>
> >>>>>> Good point.
> >>>>>> I think we need to surround efa_process_comp_eqe() with an rcu_read_lock() and
> >>>>>> have a synchronize_rcu() after removing it from the xarray in
> >>>>>> destroy_cq.
> >>>>>
> >>>>> Try to avoid synchronize_rcu()
> >>>>
> >>>> I don't see how that's possible?
> >>>
> >>> Usually people use call_rcu() instead
> >>
> >> Oh nice, thanks.
> >>
> >> I think the code would be much simpler using synchronize_rcu(), and the
> >> destroy_cq flow is usually on the cold path anyway. I also prefer to be certain
> >> that the CQ is freed once the destroy verb returns and not rely on the callback
> >> scheduling.
> > 
> > I would not be happy to see synchronize_rcu on uverbs destroy
> > functions, it is too easy to DOS the kernel with that.
> 
> OK, but isn't the fact that the uverb can return before the CQ is actually
> destroyed problematic?

Yes, you can't allow that, something other than RCU needs to prevent
that

> Maybe it's an extreme corner case, but if I created max_cq CQs, destroyed one,
> and try to create another one, it is not guaranteed that the create operation
> would succeed - even though the destroy has finished.

More importantly a driver cannot call completion callbacks once
destroy cq has returned.

Jason