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

From: Gal Pressman <galpress@amazon.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Doug Ledford <dledford@redhat.com>, <linux-rdma@vger.kernel.org>,
	Alexander Matushevsky <matua@amazon.com>,
	Firas JahJah <firasj@amazon.com>,
	Yossi Leybovich <sleybo@amazon.com>
Subject: Re: [PATCH for-next 4/4] RDMA/efa: CQ notifications
Date: Thu, 2 Sep 2021 10:03:16 +0300	[thread overview]
Message-ID: <d1b2dc01-5a42-371e-c4b6-2f9b3425f5b6@amazon.com> (raw)
In-Reply-To: <20210901153659.GL1721383@nvidia.com>

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?

>> 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()?

Sure, I wasn't sure if it's OK to nest rcu_read_lock() calls.

>> Do you have any better ideas?
> 
> Other common alternative is to use the xa_lock(), but that doesn't
> seem suitable for an EQ

Right, I think I'll go with the rcu approach.