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=-0.9 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 B7A37C67839 for ; Thu, 13 Dec 2018 19:46:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 813092086D for ; Thu, 13 Dec 2018 19:46:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 813092086D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728584AbeLMTqR (ORCPT ); Thu, 13 Dec 2018 14:46:17 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56930 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727684AbeLMTqR (ORCPT ); Thu, 13 Dec 2018 14:46:17 -0500 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 25A0D3002891; Thu, 13 Dec 2018 19:46:16 +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 345496179D; Thu, 13 Dec 2018 19:46:14 +0000 (UTC) Subject: Re: WARNING: locking bug in lock_downgrade To: 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, peterz@infradead.org, "mingo@redhat.com" , boqun.feng@gmail.com References: <00000000000043ae20057b974f14@google.com> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; 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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: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Thu, 13 Dec 2018 19:46:16 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/12/2018 08:14 PM, Yang Shi wrote: > Cc'ed Peter, Ingo and Waiman. > > > It took me a few days to look into this warning, but I got lost in > lockdep code. > > > The problem is the commit dd2283f2605e ("mm: mmap: zap pages with read > mmap_sem in munmap") does an optimization for munmap by downgrading > write mmap_sem to read before zapping pages. But, lockdep reports > downgrading a read lock. > > > I'm pretty sure mmap_sem is held as write before downgrade_write() is > called in the patch. And, there are 4 places which may downgrade a > mmap_sem: > >     - munmap > >     - mremap > >     - brk > >     - clear_refs_write (fs/proc/task_mmu.c) > > > The first three come from my patches, and they just do: > down_write_killable() -> .. -> downgrade_write(). > > But the last one is a little bit more complicated, it does down_read() > ->.. -> up_read() ->.. -> down_write_killable() ->.. -> > downgrade_write(). > > And, the last one may be called from any process to touch the other > processes' mmap_sem. > > > 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