From mboxrd@z Thu Jan 1 00:00:00 1970 From: sagi@grimberg.me (Sagi Grimberg) Date: Wed, 20 Jul 2016 11:47:12 +0300 Subject: [PATCH RFC 0/3] iwarp device removal deadlock fix In-Reply-To: References: Message-ID: <578F3A90.1000208@grimberg.me> > This RFC series attempts to address the deadlock issue discovered > while testing nvmf/rdma handling rdma device removal events from > the rdma_cm. Thanks for doing this Steve! > For a discussion of the deadlock that can happen, see > > http://lists.infradead.org/pipermail/linux-nvme/2016-July/005440.html. > > For my description of the deadlock itself, see this post in the above thread: > > http://lists.infradead.org/pipermail/linux-nvme/2016-July/005465.html > > In a nutshell, iw_cxgb4 and the iw_cm block during qp/cm_id destruction > until all references are removed. This combined with the iwarp CM passing > disconnect events up to the rdma_cm during disconnect and/or qp/cm_id destruction > leads to a deadlock. > > My proposed solution is to remove the need for iw_cxgb4 and iw_cm to > block during object destruction for the recnts to reach 0, but rather to > let the freeing of the object memory be deferred when the last deref is > done. This allows all the qps/cm_ids to be destroyed without blocking, and > all the object memory freeing ends up happinging when the application's > device_remove event handler function returns to the rdma_cm. This sounds like a very good approach moving forward. > Sean, I was hoping you could have a look at the iwcm.c patch particularly, > to tell my why its broken. :) I spent some time trying to figure out > why we really need the CALLBACK_DESTROY flag, but I concluded it really > isn't needed. The one side effect I see with my change, is that the > application could possibly get a cm_id event after it has destroyed the > cm_id. There probably is a way to discard events that have a reference > on the cm_id but get processed after the app has destoyed the cm_id by > having a new flag indicating "destroyed by app". That sounds easy enough. Does this mean that iwcm relies on the driver to do this or is it inter-operable with the existing logic? If not this will need to take care of all the iWARP drivers. From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sagi Grimberg Subject: Re: [PATCH RFC 0/3] iwarp device removal deadlock fix Date: Wed, 20 Jul 2016 11:47:12 +0300 Message-ID: <578F3A90.1000208@grimberg.me> References: Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Steve Wise , linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Cc: sean.hefty-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org, mlin-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, hch-jcswGhMUV9g@public.gmane.org, linux-nvme-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org List-Id: linux-rdma@vger.kernel.org > This RFC series attempts to address the deadlock issue discovered > while testing nvmf/rdma handling rdma device removal events from > the rdma_cm. Thanks for doing this Steve! > For a discussion of the deadlock that can happen, see > > http://lists.infradead.org/pipermail/linux-nvme/2016-July/005440.html. > > For my description of the deadlock itself, see this post in the above thread: > > http://lists.infradead.org/pipermail/linux-nvme/2016-July/005465.html > > In a nutshell, iw_cxgb4 and the iw_cm block during qp/cm_id destruction > until all references are removed. This combined with the iwarp CM passing > disconnect events up to the rdma_cm during disconnect and/or qp/cm_id destruction > leads to a deadlock. > > My proposed solution is to remove the need for iw_cxgb4 and iw_cm to > block during object destruction for the recnts to reach 0, but rather to > let the freeing of the object memory be deferred when the last deref is > done. This allows all the qps/cm_ids to be destroyed without blocking, and > all the object memory freeing ends up happinging when the application's > device_remove event handler function returns to the rdma_cm. This sounds like a very good approach moving forward. > Sean, I was hoping you could have a look at the iwcm.c patch particularly, > to tell my why its broken. :) I spent some time trying to figure out > why we really need the CALLBACK_DESTROY flag, but I concluded it really > isn't needed. The one side effect I see with my change, is that the > application could possibly get a cm_id event after it has destroyed the > cm_id. There probably is a way to discard events that have a reference > on the cm_id but get processed after the app has destoyed the cm_id by > having a new flag indicating "destroyed by app". That sounds easy enough. Does this mean that iwcm relies on the driver to do this or is it inter-operable with the existing logic? If not this will need to take care of all the iWARP drivers. -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html