From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 19B53C433F5 for ; Sun, 5 Sep 2021 10:54:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E47BD60EE1 for ; Sun, 5 Sep 2021 10:54:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237534AbhIEKz6 (ORCPT ); Sun, 5 Sep 2021 06:55:58 -0400 Received: from mail.kernel.org ([198.145.29.99]:36216 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230145AbhIEKz6 (ORCPT ); Sun, 5 Sep 2021 06:55:58 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id A5FFD60ED8; Sun, 5 Sep 2021 10:54:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1630839295; bh=laS9vZm1paQYFsAHzFHvXJva3O6AxlrQujJoGe81QvI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BQ0RmbmeyKd/eEliQv0JjXHT59JKKrwUKRMZh1o0QQB4rBlYyfV5fm6kZqcBJSUon EOGVaY/sEVk6d9r6mfImB0PIFKWXWIh4erQd5WCtRiRPlXW1ssbeRaIwh5xFWegGuJ BPZuyz6w1FIBFlW1EcMtSeaxZPVx6901G2Iypd+up91DunvfACgg/89eZywdt3x+S8 xqnCbeYgun7F7vWTDuxq/gcJYC5lkIKzcIhvmIouilYM27984oMvaba47hJyIoEedb nwRqhRAUvBWrO+cIMhy/sWW0r9NMIouKrlXES12tXOt2WNEDGJxYwNszxRtDpythhi smLW+tYq0n4Gw== Date: Sun, 5 Sep 2021 13:54:51 +0300 From: Leon Romanovsky To: Gal Pressman Cc: Jason Gunthorpe , Doug Ledford , linux-rdma@vger.kernel.org, Alexander Matushevsky , Firas JahJah , Yossi Leybovich Subject: Re: [PATCH for-next 4/4] RDMA/efa: CQ notifications Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org 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(). Thanks > > call_rcu() don't really fit this use case.