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

[Gal Pressman <galpress@amazon.com>]

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.