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

[Gal Pressman <galpress@amazon.com>]

On 05/09/2021 13:54, Leon Romanovsky wrote:
> 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().

The irq flow doesn't call rdma_restrack_get() so I'm not sure how the
wait_for_completion() makes a difference here.
And if it does then the code is fine as is? There's nothing the call_rcu() needs
to do.