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

[Gal Pressman <galpress@amazon.com>]

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

Do you have any better ideas?