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

[Leon Romanovsky <leon@kernel.org>]

On Sun, Sep 05, 2021 at 01:45:41PM +0300, Gal Pressman wrote:
> On 05/09/2021 10:59, Leon Romanovsky wrote:
> > On Sun, Sep 05, 2021 at 10:25:17AM +0300, Gal Pressman wrote:
> >> On 02/09/2021 18:41, Jason Gunthorpe wrote:
> >>> 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.
> >>
> >> So how is having some kind of synchronization to wait for the call_rcu()
> >> callback to finish different than using synchronize_rcu()? We'll have to wait
> >> for the readers to finish before returning.
> > 
> > Why do you need to do anything special in addition to nullify
> > completion callback which will ensure that no new readers are
> > coming and call_rcu to make sure that existing readers finished?
> 
> I ensure there are no new readers by removing the CQ from the xarray.
> Then I must wait for all existing readers before returning from efa_destroy_cq
> and freeing the cq struct (which is done by ib_core).

IB/core calls to rdma_restrack_del() which wait_for_completion() before
freeing CQ and returning to the users. You don't need to wait in
efa_destroy_cq().

Thanks

> 
> call_rcu() don't really fit this use case.