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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 2F76BC43387 for ; Wed, 9 Jan 2019 22:46:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E7AD8206B7 for ; Wed, 9 Jan 2019 22:46:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726541AbfAIWqI (ORCPT ); Wed, 9 Jan 2019 17:46:08 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34516 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726217AbfAIWqH (ORCPT ); Wed, 9 Jan 2019 17:46:07 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6732288308; Wed, 9 Jan 2019 22:46:06 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-223.bos.redhat.com [10.18.17.223]) by smtp.corp.redhat.com (Postfix) with ESMTP id 259C31019633; Wed, 9 Jan 2019 22:46:04 +0000 (UTC) Subject: Re: WARNING: locking bug in lock_downgrade To: Tetsuo Handa , peterz@infradead.org, "mingo@redhat.com" Cc: Yang Shi , syzbot , akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux@dominikbrodowski.net, mhocko@suse.com, rientjes@google.com, syzkaller-bugs@googlegroups.com, vbabka@suse.cz, boqun.feng@gmail.com References: <00000000000043ae20057b974f14@google.com> <69273d51-c129-6b0f-35eb-d98655476ff9@redhat.com> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; prefer-encrypt=mutual; keydata= xsFNBFgsZGsBEAC3l/RVYISY3M0SznCZOv8aWc/bsAgif1H8h0WPDrHnwt1jfFTB26EzhRea XQKAJiZbjnTotxXq1JVaWxJcNJL7crruYeFdv7WUJqJzFgHnNM/upZuGsDIJHyqBHWK5X9ZO jRyfqV/i3Ll7VIZobcRLbTfEJgyLTAHn2Ipcpt8mRg2cck2sC9+RMi45Epweu7pKjfrF8JUY r71uif2ThpN8vGpn+FKbERFt4hW2dV/3awVckxxHXNrQYIB3I/G6mUdEZ9yrVrAfLw5M3fVU CRnC6fbroC6/ztD40lyTQWbCqGERVEwHFYYoxrcGa8AzMXN9CN7bleHmKZrGxDFWbg4877zX 0YaLRypme4K0ULbnNVRQcSZ9UalTvAzjpyWnlnXCLnFjzhV7qsjozloLTkZjyHimSc3yllH7 VvP/lGHnqUk7xDymgRHNNn0wWPuOpR97J/r7V1mSMZlni/FVTQTRu87aQRYu3nKhcNJ47TGY evz/U0ltaZEU41t7WGBnC7RlxYtdXziEn5fC8b1JfqiP0OJVQfdIMVIbEw1turVouTovUA39 Qqa6Pd1oYTw+Bdm1tkx7di73qB3x4pJoC8ZRfEmPqSpmu42sijWSBUgYJwsziTW2SBi4hRjU h/Tm0NuU1/R1bgv/EzoXjgOM4ZlSu6Pv7ICpELdWSrvkXJIuIwARAQABzR9Mb25nbWFuIExv bmcgPGxsb25nQHJlZGhhdC5jb20+wsF/BBMBAgApBQJYLGRrAhsjBQkJZgGABwsJCAcDAgEG FQgCCQoLBBYCAwECHgECF4AACgkQbjBXZE7vHeYwBA//ZYxi4I/4KVrqc6oodVfwPnOVxvyY oKZGPXZXAa3swtPGmRFc8kGyIMZpVTqGJYGD9ZDezxpWIkVQDnKM9zw/qGarUVKzElGHcuFN ddtwX64yxDhA+3Og8MTy8+8ZucM4oNsbM9Dx171bFnHjWSka8o6qhK5siBAf9WXcPNogUk4S fMNYKxexcUayv750GK5E8RouG0DrjtIMYVJwu+p3X1bRHHDoieVfE1i380YydPd7mXa7FrRl 7unTlrxUyJSiBc83HgKCdFC8+ggmRVisbs+1clMsK++ehz08dmGlbQD8Fv2VK5KR2+QXYLU0 rRQjXk/gJ8wcMasuUcywnj8dqqO3kIS1EfshrfR/xCNSREcv2fwHvfJjprpoE9tiL1qP7Jrq 4tUYazErOEQJcE8Qm3fioh40w8YrGGYEGNA4do/jaHXm1iB9rShXE2jnmy3ttdAh3M8W2OMK 4B/Rlr+Awr2NlVdvEF7iL70kO+aZeOu20Lq6mx4Kvq/WyjZg8g+vYGCExZ7sd8xpncBSl7b3 99AIyT55HaJjrs5F3Rl8dAklaDyzXviwcxs+gSYvRCr6AMzevmfWbAILN9i1ZkfbnqVdpaag QmWlmPuKzqKhJP+OMYSgYnpd/vu5FBbc+eXpuhydKqtUVOWjtp5hAERNnSpD87i1TilshFQm TFxHDzbOwU0EWCxkawEQALAcdzzKsZbcdSi1kgjfce9AMjyxkkZxcGc6Rhwvt78d66qIFK9D Y9wfcZBpuFY/AcKEqjTo4FZ5LCa7/dXNwOXOdB1Jfp54OFUqiYUJFymFKInHQYlmoES9EJEU yy+2ipzy5yGbLh3ZqAXyZCTmUKBU7oz/waN7ynEP0S0DqdWgJnpEiFjFN4/ovf9uveUnjzB6 lzd0BDckLU4dL7aqe2ROIHyG3zaBMuPo66pN3njEr7IcyAL6aK/IyRrwLXoxLMQW7YQmFPSw drATP3WO0x8UGaXlGMVcaeUBMJlqTyN4Swr2BbqBcEGAMPjFCm6MjAPv68h5hEoB9zvIg+fq M1/Gs4D8H8kUjOEOYtmVQ5RZQschPJle95BzNwE3Y48ZH5zewgU7ByVJKSgJ9HDhwX8Ryuia 79r86qZeFjXOUXZjjWdFDKl5vaiRbNWCpuSG1R1Tm8o/rd2NZ6l8LgcK9UcpWorrPknbE/pm MUeZ2d3ss5G5Vbb0bYVFRtYQiCCfHAQHO6uNtA9IztkuMpMRQDUiDoApHwYUY5Dqasu4ZDJk bZ8lC6qc2NXauOWMDw43z9He7k6LnYm/evcD+0+YebxNsorEiWDgIW8Q/E+h6RMS9kW3Rv1N qd2nFfiC8+p9I/KLcbV33tMhF1+dOgyiL4bcYeR351pnyXBPA66ldNWvABEBAAHCwWUEGAEC AA8FAlgsZGsCGwwFCQlmAYAACgkQbjBXZE7vHeYxSQ/+PnnPrOkKHDHQew8Pq9w2RAOO8gMg 9Ty4L54CsTf21Mqc6GXj6LN3WbQta7CVA0bKeq0+WnmsZ9jkTNh8lJp0/RnZkSUsDT9Tza9r GB0svZnBJMFJgSMfmwa3cBttCh+vqDV3ZIVSG54nPmGfUQMFPlDHccjWIvTvyY3a9SLeamaR jOGye8MQAlAD40fTWK2no6L1b8abGtziTkNh68zfu3wjQkXk4kA4zHroE61PpS3oMD4AyI9L 7A4Zv0Cvs2MhYQ4Qbbmafr+NOhzuunm5CoaRi+762+c508TqgRqH8W1htZCzab0pXHRfywtv 0P+BMT7vN2uMBdhr8c0b/hoGqBTenOmFt71tAyyGcPgI3f7DUxy+cv3GzenWjrvf3uFpxYx4 yFQkUcu06wa61nCdxXU/BWFItryAGGdh2fFXnIYP8NZfdA+zmpymJXDQeMsAEHS0BLTVQ3+M 7W5Ak8p9V+bFMtteBgoM23bskH6mgOAw6Cj/USW4cAJ8b++9zE0/4Bv4iaY5bcsL+h7TqQBH Lk1eByJeVooUa/mqa2UdVJalc8B9NrAnLiyRsg72Nurwzvknv7anSgIkL+doXDaG21DgCYTD wGA5uquIgb8p3/ENgYpDPrsZ72CxVC2NEJjJwwnRBStjJOGQX4lV1uhN1XsZjBbRHdKF2W9g weim8xU= Organization: Red Hat Message-ID: <864e2d6b-f471-cc04-311f-473da43b409a@redhat.com> Date: Wed, 9 Jan 2019 17:46:03 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Wed, 09 Jan 2019 22:46:06 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/09/2019 09:18 AM, Tetsuo Handa wrote: > On 2018/12/14 4:46, Waiman Long wrote: >> On 12/12/2018 08:14 PM, Yang Shi wrote: >>> By looking into lockdep code, I'm not sure if lockdep may get confused >>> by such sequence or not? >>> >>> >>> Any hint is appreciated. >>> >>> >>> Regards, >>> >>> Yang >> The warning was printed because hlock->read was set when doing the >> downgrade_write(). So it is either downgrade_write() was called a second >> time or a read lock was held originally. It is hard to tell what is the >> root cause without a reproducer. >> >> Cheers, >> Longman >> > Comparing with output from > > struct rw_semaphore *sem = ¤t->mm->mmap_sem; > > down_write(sem); > pr_warn("mmap_sem: count=%ld current=%px, owner=%px\n", atomic_long_read(&sem->count), current, READ_ONCE(sem->owner)); > /* mmap_sem: count=-4294967295 current=ffff88813095ca80, owner=ffff88813095ca80 */ > downgrade_write(sem); > pr_warn("mmap_sem: count=%ld current=%px, owner=%px\n", atomic_long_read(&sem->count), current, READ_ONCE(sem->owner)); > /* mmap_sem: count=1 current=ffff88813095ca80, owner=ffff88813095ca83 */ > up_read(sem); > pr_warn("mmap_sem: count=%ld current=%px, owner=%px\n", atomic_long_read(&sem->count), current, READ_ONCE(sem->owner)); > /* mmap_sem: count=0 current=ffff88813095ca80, owner=0000000000000003 */ The behavior is correct. The current code will leave the reader task structure pointer in owner even if it is a read lock. You have to look at bit 0 to know if the owner is a reader or writer. > what we got with debug printk() patch > > https://syzkaller.appspot.com/text?tag=CrashLog&x=169dbb9b400000 > > [ 2580.337550][ T3645] mmap_sem: hlock->read=1 count=-4294967295 current=ffff888050e04140, owner=ffff888050e04140 > [ 2580.353526][ T3645] ------------[ cut here ]------------ > [ 2580.367859][ T3645] downgrading a read lock > [ 2580.367935][ T3645] WARNING: CPU: 1 PID: 3645 at kernel/locking/lockdep.c:3572 lock_downgrade+0x35d/0xbe0 > [ 2580.382206][ T3645] Kernel panic - not syncing: panic_on_warn set ... > > https://syzkaller.appspot.com/text?tag=CrashLog&x=1542da4f400000 > > [ 386.342585][T16698] mmap_sem: hlock->read=1 count=-4294967295 current=ffff8880512ae180, owner=ffff8880512ae180 > [ 386.348586][T16698] ------------[ cut here ]------------ > [ 386.357203][T16698] downgrading a read lock > [ 386.357294][T16698] WARNING: CPU: 1 PID: 16698 at kernel/locking/lockdep.c:3572 lock_downgrade+0x35d/0xbe0 > [ 386.372148][T16698] Kernel panic - not syncing: panic_on_warn set ... > > indicates that lockdep is saying that "current->mm->mmap_sem is held for read" > while "struct rw_semaphore" is saying that "current->mm->mmap_sem is held for write". > Something made lockdep confused. Possibly a lockdep bug. > It could be a bug in lockdep regarding downgrade. Someone else has reported similar problem before. -Longman