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

[Jason Gunthorpe <jgg@nvidia.com>]

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()

> Though I'd like to avoid copy-pasting xa_load() in order to use the
> advanced xas_load() function.

Why would you need that? Just call xa_load() while holding the
rcu_read_lock() if that is what you want. Can you put the whole
function under the rcu_read_lock()?

> Do you have any better ideas?

Other common alternative is to use the xa_lock(), but that doesn't
seem suitable for an EQ

Jason