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=-0.6 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID 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 2EBB8C43140 for ; Thu, 21 Jun 2018 03:35:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B907520875 for ; Thu, 21 Jun 2018 03:35:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="NklE6PJ5" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B907520875 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org 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 S1754423AbeFUDf3 (ORCPT ); Wed, 20 Jun 2018 23:35:29 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:57236 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754166AbeFUDf2 (ORCPT ); Wed, 20 Jun 2018 23:35:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=9rN4WPOaDAdQrHZKtVSKwfnGHageL7nl3Z8DGtCigv0=; b=NklE6PJ5ubwSxjlLz4NSlG4/L myzDFV0Zy2YerYDp+8gnjubPKYPbuzkSH8SsDAdgWDFT3dDQeW0/D8DGCcrpHUVCXo4GmDmd8rI4P HPMbvJFJFMrF0n6BNbdS0w38UD4II6e5dQC6XGe2UOSntdBgPsrHmXvdYEncxZ8XiEZnx1tkZ5QGQ 9awbJGAEhixHzwzzIDppvohMsEpmm/xBF2NKPhc7hVVDdy7++mAQ3ea26ZTT50+YfnR3JniawcJxZ YeyryO3+nYrYxrmmQ8GNODCz9Svzl3jFP2kGrhrsWWtz2YVvodW4MeC8bG9kejZ7XTgJD8NhTrvY5 hdCnAZjfg==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1fVqNP-00088l-BD; Thu, 21 Jun 2018 03:35:15 +0000 Date: Wed, 20 Jun 2018 20:35:15 -0700 From: Matthew Wilcox To: Jia-Ju Bai Cc: axboe@kernel.dk, akpm@linux-foundation.or, jack@suse.cz, zhangweiping@didichuxing.com, sergey.senozhatsky@gmail.com, andriy.shevchenko@linux.intel.com, christophe.jaillet@wanadoo.fr, aryabinin@virtuozzo.com, linux-mm@kvack.org, Linux Kernel Mailing List Subject: Re: [BUG] mm: backing-dev: a possible sleep-in-atomic-context bug in cgwb_create() Message-ID: <20180621033515.GA12608@bombadil.infradead.org> References: <626acba3-c565-7e05-6c8b-0d100ff645c5@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <626acba3-c565-7e05-6c8b-0d100ff645c5@gmail.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 21, 2018 at 11:02:58AM +0800, Jia-Ju Bai wrote: > The kernel may sleep with holding a spinlock. > The function call path (from bottom to top) in Linux-4.16.7 is: > > [FUNC] schedule > lib/percpu-refcount.c, 222: > schedule in __percpu_ref_switch_mode > lib/percpu-refcount.c, 339: > __percpu_ref_switch_mode in percpu_ref_kill_and_confirm > ./include/linux/percpu-refcount.h, 127: > percpu_ref_kill_and_confirm in percpu_ref_kill > mm/backing-dev.c, 545: > percpu_ref_kill in cgwb_kill > mm/backing-dev.c, 576: > cgwb_kill in cgwb_create > mm/backing-dev.c, 573: > _raw_spin_lock_irqsave in cgwb_create > > This bug is found by my static analysis tool (DSAC-2) and checked by my > code review. I disagree with your code review. * If the previous ATOMIC switching hasn't finished yet, wait for * its completion. If the caller ensures that ATOMIC switching * isn't in progress, this function can be called from any context. I believe cgwb_kill is always called under the spinlock, so we will never sleep because the percpu ref will never be switching to atomic mode. This is complex and subtle, so I could be wrong.