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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6832CC3525A for ; Wed, 26 Jan 2022 01:09:07 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 747646B0075; Tue, 25 Jan 2022 20:09:06 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6F71C6B007B; Tue, 25 Jan 2022 20:09:06 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5E6486B007D; Tue, 25 Jan 2022 20:09:06 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay026.a.hostedemail.com [64.99.140.26]) by kanga.kvack.org (Postfix) with ESMTP id 4D0DD6B0075 for ; Tue, 25 Jan 2022 20:09:06 -0500 (EST) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 0649621FEA for ; Wed, 26 Jan 2022 01:09:06 +0000 (UTC) X-FDA: 79070654292.02.DCF4520 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf31.hostedemail.com (Postfix) with ESMTP id 8F03C20002 for ; Wed, 26 Jan 2022 01:09:05 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id EDC61B81A23; Wed, 26 Jan 2022 01:09:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 43C1BC340E0; Wed, 26 Jan 2022 01:09:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1643159342; bh=D6r77K7QjaFLL+Ex50/C3+VaPdgBBea9XHVhawpPP5E=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=cfIVlgBxdxoEmDlzxN8MLdZiDeqfVMs3F9R44x1EEDi+FAcc219spoDmj/bUz+Llv h5JdE3MYN7DlL3+6DvlIKyfve25qK4t/2jBcjqDCi5sYv2yBNTNSHibXSD3wYTXTIt MuNS0xHZw3u41EGgehRxBHDemS/y4UWPqGR1HW3E= Date: Tue, 25 Jan 2022 17:09:00 -0800 From: Andrew Morton To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Baoquan He , Vivek Goyal , Dave Young , "Paul E. McKenney" , Josh Triplett , Peter Zijlstra , Boqun Feng Subject: Re: [PATCH v2] proc/vmcore: fix possible deadlock on concurrent mmap and read Message-Id: <20220125170900.472fdb649312e77a4a60d9da@linux-foundation.org> In-Reply-To: <20220119193417.100385-1-david@redhat.com> References: <20220119193417.100385-1-david@redhat.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Stat-Signature: 49zcfqgsfze937knitg7yb3f6jzagqfs X-Rspam-User: nil Authentication-Results: imf31.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=cfIVlgBx; spf=pass (imf31.hostedemail.com: domain of akpm@linux-foundation.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 8F03C20002 X-HE-Tag: 1643159345-156012 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 19 Jan 2022 20:34:17 +0100 David Hildenbrand wrote: > Lockdep noticed that there is chance for a deadlock if we have > concurrent mmap, concurrent read, and the addition/removal of a > callback. > > As nicely explained by Boqun: > > " > Lockdep warned about the above sequences because rw_semaphore is a fair > read-write lock, and the following can cause a deadlock: > > TASK 1 TASK 2 TASK 3 > ====== ====== ====== > down_write(mmap_lock); > down_read(vmcore_cb_rwsem) > down_write(vmcore_cb_rwsem); // blocked > down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness > down_read(mmap_lock); // blocked I'm wondering about cc:stable. It's hard to believe that this is likely to be observed in real life. But the ongoing reports of lockdep splats will be irritating.