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Sun, 5 Sep 2021 11:05:20 +0000 Subject: Re: [PATCH for-next 4/4] RDMA/efa: CQ notifications To: Leon Romanovsky CC: Jason Gunthorpe , Doug Ledford , , Alexander Matushevsky , Firas JahJah , Yossi Leybovich References: <20210901153659.GL1721383@nvidia.com> <20210902130255.GR1721383@nvidia.com> <3a5fb37a-dd72-e322-f7c6-790ee4e04efa@amazon.com> <20210902151029.GV1721383@nvidia.com> <20210902154124.GX1721383@nvidia.com> <9ffde1c4-d748-0091-0d7d-b2e2eb63aa51@amazon.com> From: Gal Pressman Message-ID: <2231dfa4-2f99-5187-fa83-56052dad9979@amazon.com> Date: Sun, 5 Sep 2021 14:05:15 +0300 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.43.160.66] X-ClientProxiedBy: EX13D39UWA002.ant.amazon.com (10.43.160.20) To EX13D19EUB003.ant.amazon.com (10.43.166.69) Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org 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.