From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Cyrus-Session-Id: sloti22d1t05-1079496-1521673857-2-12349592576003400985 X-Sieve: CMU Sieve 3.0 X-Spam-known-sender: no ("Email failed DMARC policy for domain") X-Spam-score: 0.0 X-Spam-hits: BAYES_00 -1.9, HEADER_FROM_DIFFERENT_DOMAINS 0.25, ME_NOAUTH 0.01, RCVD_IN_DNSWL_HI -5, T_RP_MATCHES_RCVD -0.01, LANGUAGES en, BAYES_USED global, SA_VERSION 3.4.0 X-Spam-source: IP='209.132.180.67', Host='vger.kernel.org', Country='CN', FromHeader='com', MailFrom='org', XOriginatingCountry='UNK' X-Spam-charsets: plain='utf-8' X-IgnoreVacation: yes ("Email failed DMARC policy for domain") X-Resolved-to: greg@kroah.com X-Delivered-to: greg@kroah.com X-Mail-from: stable-owner@vger.kernel.org ARC-Seal: i=1; a=rsa-sha256; cv=none; d=messagingengine.com; s=arctest; t=1521673857; b=JbCg6pQnj6pfyaj5tUZ7O66L4f114EGFnbw1ijlNtAMo09P 9uRgblBD7Rl+ZFgghVKSuo0yTXbZ5Y5pb/OvDlEoIie+xLrub3ZH1lyLc11lH9jA eBeiKOOtpOUUP1JqFP/8sajMixp8SrxdIxtvIM/WxjNrh8BpqnabZY22tnOUY6nj derqT3PuoM83xIhRvsh5jxEMarpm/tOPV7FhR4iKF/5sBbY3sJh8t+L1eZ20dtrm vFxXzgsk3WChv9VofVdwFPqG2wlBwAzpwARm1GEgQEUS+nSt1dO4EEqpMUW0sC0A 5oolD+K843XGlSCAg0n3+NcOt8vXzAHu8RdLMcA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=subject:to:cc:references:from:message-id :date:mime-version:in-reply-to:content-type :content-transfer-encoding:sender:list-id; s=arctest; t= 1521673857; bh=bdLQND42A37IXT/nWCPE0qAc71OxX2r303DU5dhrpWg=; b=V YrCgaab3s6AopLx9Sqx2r39h/vEgIU+BwiSSBpEQHCv1zq6al5LR9jqmvCE5Cksb HAot53gX3dfDTA8qoaedcfRbBhyOl1WnY6op0M/3nQeif2h9sCSJR257bNgUR0mD GjXLDAv6vQ0QxNcILNl7j6Vexdjnnb8JEM29ew7y9Lpkb2iW+uiD9ngOyAkCc+ku Uk9PIMHkQiIKCkHdOO0JVa8PpfIFmIXQOdfBZ1zLu9NWOLW7qUCS76VnoGTqyYsg rSc1Jow6Fd93YMGXQ4FrW2bTOpaRAuQ/RYQgCN83+JGfXgAvKsMmVVBnBVwYjXWl S9Llyz6Tx7lQlyYeol3SA== ARC-Authentication-Results: i=1; mx5.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=fail (p=none,has-list-id=yes,d=none) header.from=nvidia.com; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=stable-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=nvidia.com header.result=pass header_is_org_domain=yes; x-vs=clean score=-100 state=0 Authentication-Results: mx5.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=fail (p=none,has-list-id=yes,d=none) header.from=nvidia.com; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=stable-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=nvidia.com header.result=pass header_is_org_domain=yes; x-vs=clean score=-100 state=0 X-ME-VSCategory: clean Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754319AbeCUXKk convert rfc822-to-8bit (ORCPT ); Wed, 21 Mar 2018 19:10:40 -0400 Received: from hqemgate15.nvidia.com ([216.228.121.64]:14683 "EHLO hqemgate15.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754279AbeCUXKe (ORCPT ); Wed, 21 Mar 2018 19:10:34 -0400 X-PGP-Universal: processed; by hqpgpgate102.nvidia.com on Wed, 21 Mar 2018 16:10:28 -0700 Subject: Re: [PATCH 03/15] mm/hmm: HMM should have a callback before MM is destroyed v2 To: Jerome Glisse CC: , Andrew Morton , , Ralph Campbell , , Evgeny Baskakov , Mark Hairgrove References: <20180320020038.3360-1-jglisse@redhat.com> <20180320020038.3360-4-jglisse@redhat.com> <20180321180342.GE3214@redhat.com> <788cf786-edbf-ab43-af0d-abbe9d538757@nvidia.com> <20180321224620.GH3214@redhat.com> X-Nvconfidentiality: public From: John Hubbard Message-ID: <3f4e78a1-5a88-399d-e134-497229c42707@nvidia.com> Date: Wed, 21 Mar 2018 16:10:32 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <20180321224620.GH3214@redhat.com> X-Originating-IP: [10.110.48.28] X-ClientProxiedBy: HQMAIL108.nvidia.com (172.18.146.13) To HQMAIL107.nvidia.com (172.20.187.13) Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8BIT Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On 03/21/2018 03:46 PM, Jerome Glisse wrote: > On Wed, Mar 21, 2018 at 03:16:04PM -0700, John Hubbard wrote: >> On 03/21/2018 11:03 AM, Jerome Glisse wrote: >>> On Tue, Mar 20, 2018 at 09:14:34PM -0700, John Hubbard wrote: >>>> On 03/19/2018 07:00 PM, jglisse@redhat.com wrote: >>>>> From: Ralph Campbell >> >> >> >>>> Hi Jerome, >>>> >>>> This presents a deadlock problem (details below). As for solution ideas, >>>> Mark Hairgrove points out that the MMU notifiers had to solve the >>>> same sort of problem, and part of the solution involves "avoid >>>> holding locks when issuing these callbacks". That's not an entire >>>> solution description, of course, but it seems like a good start. >>>> >>>> Anyway, for the deadlock problem: >>>> >>>> Each of these ->release callbacks potentially has to wait for the >>>> hmm_invalidate_range() callbacks to finish. That is not shown in any >>>> code directly, but it's because: when a device driver is processing >>>> the above ->release callback, it has to allow any in-progress operations >>>> to finish up (as specified clearly in your comment documentation above). >>>> >>>> Some of those operations will invariably need to do things that result >>>> in page invalidations, thus triggering the hmm_invalidate_range() callback. >>>> Then, the hmm_invalidate_range() callback tries to acquire the same >>>> hmm->mirrors_sem lock, thus leading to deadlock: >>>> >>>> hmm_invalidate_range(): >>>> // ... >>>> down_read(&hmm->mirrors_sem); >>>> list_for_each_entry(mirror, &hmm->mirrors, list) >>>> mirror->ops->sync_cpu_device_pagetables(mirror, action, >>>> start, end); >>>> up_read(&hmm->mirrors_sem); >>> >>> That is just illegal, the release callback is not allowed to trigger >>> invalidation all it does is kill all device's threads and stop device >>> page fault from happening. So there is no deadlock issues. I can re- >>> inforce the comment some more (see [1] for example on what it should >>> be). >> >> That rule is fine, and it is true that the .release callback will not >> directly trigger any invalidations. However, the problem is in letting >> any *existing* outstanding operations finish up. We have to let >> existing operations "drain", in order to meet the requirement that >> everything is done when .release returns. >> >> For example, if a device driver thread is in the middle of working through >> its fault buffer, it will call migrate_vma(), which will in turn unmap >> pages. That will cause an hmm_invalidate_range() callback, which tries >> to take hmm->mirrors_sems, and we deadlock. >> >> There's no way to "kill" such a thread while it's in the middle of >> migrate_vma(), you have to let it finish up. >> >>> Also it is illegal for the sync callback to trigger any mmu_notifier >>> callback. I thought this was obvious. The sync callback should only >>> update device page table and do _nothing else_. No way to make this >>> re-entrant. >> >> That is obvious, yes. I am not trying to say there is any problem with >> that rule. It's the "drain outstanding operations during .release", >> above, that is the real problem. > > Maybe just relax the release callback wording, it should stop any > more processing of fault buffer but not wait for it to finish. In > nouveau code i kill thing but i do not wait hence i don't deadlock. But you may crash, because that approach allows .release to finish up, thus removing the mm entirely, out from under (for example) a migrate_vma call--or any other call that refers to the mm. It doesn't seem too hard to avoid the problem, though: maybe we can just drop the lock while doing the mirror->ops->release callback. There are a few ways to do this, but one example is: -- take the lock, -- copy the list to a local list, deleting entries as you go, -- drop the lock, -- iterate through the local list copy and -- issue the mirror->ops->release callbacks. At this point, more items could have been added to the list, so repeat the above until the original list is empty. This is subject to a limited starvation case if mirror keep getting registered, but I think we can ignore that, because it only lasts as long as mirrors keep getting added, and then it finishes up. > > What matter is to stop any further processing. Yes some fault might > be in flight but they will serialize on various lock. Those faults in flight could already be at a point where they have taken whatever locks they need, so we don't dare let the mm get destroyed while such fault handling is in progress. So just do not > wait in the release callback, kill thing. I might have a bug where i > still fill in GPU page table in nouveau, i will check nouveau code > for that. Again, we can't "kill" a thread of execution (this would often be an interrupt bottom half context, btw) while it is, for example, in the middle of migrate_vma. I really don't believe there is a safe way to do this without draining the existing operations before .release returns, and for that, we'll need to issue the .release callbacks while not holding locks. thanks, -- John Hubbard NVIDIA > > Kill thing should also kill the channel (i don't do that in nouveau > because i am waiting on some channel patchset) but i am not sure if > hardware like it if we kill channel before stoping fault notification. > > Cheers, > Jérôme >