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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 CF7DFC433E0 for ; Thu, 11 Feb 2021 17:40:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 94F2E64E8A for ; Thu, 11 Feb 2021 17:40:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231470AbhBKRih (ORCPT ); Thu, 11 Feb 2021 12:38:37 -0500 Received: from mx2.suse.de ([195.135.220.15]:33650 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230326AbhBKQmF (ORCPT ); Thu, 11 Feb 2021 11:42:05 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613061677; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=49277//KwxbjQ405XwayXDeBcBaVqNsLWaTMf3MliEo=; b=YV5YY0wNG1juqlUn4dbLg5e38JEvoh38LhXzPWw7qoeQqD4eW6kYuna41RTqh46JN+/Mry 5awcoW5BM4BtWa4GjbgXfx6pzlWQm1G/96w08Tmo16oMjOTkVb74Y8m4+5necwrZ+bpTJk JUcMDs3qmoMDTdlggXe5mSkmOda7ZSY= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C4B87AC43; Thu, 11 Feb 2021 16:41:17 +0000 (UTC) Date: Thu, 11 Feb 2021 17:41:16 +0100 From: Michal Hocko To: Matthew Wilcox Cc: Jan Kara , Dmitry Vyukov , syzbot , Jan Kara , linux-ext4@vger.kernel.org, LKML , syzkaller-bugs , Theodore Ts'o , Linux-MM Subject: Re: possible deadlock in start_this_handle (2) Message-ID: References: <20210211104947.GL19070@quack2.suse.cz> <20210211121020.GO19070@quack2.suse.cz> <20210211125717.GH308988@casper.infradead.org> <20210211132533.GI308988@casper.infradead.org> <20210211142630.GK308988@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210211142630.GK308988@casper.infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 11-02-21 14:26:30, Matthew Wilcox wrote: > On Thu, Feb 11, 2021 at 03:20:41PM +0100, Michal Hocko wrote: > > On Thu 11-02-21 13:25:33, Matthew Wilcox wrote: > > > On Thu, Feb 11, 2021 at 02:07:03PM +0100, Michal Hocko wrote: > > > > On Thu 11-02-21 12:57:17, Matthew Wilcox wrote: > > > > > > current->flags should be always manipulated from the user context. But > > > > > > who knows maybe there is a bug and some interrupt handler is calling it. > > > > > > This should be easy to catch no? > > > > > > > > > > Why would it matter if it were? > > > > > > > > I was thinking about a clobbered state because updates to ->flags are > > > > not atomic because this shouldn't ever be updated concurrently. So maybe > > > > a racing interrupt could corrupt the flags state? > > > > > > I don't think that's possible. Same-CPU races between interrupt and > > > process context are simpler because the CPU always observes its own writes > > > in order and the interrupt handler completes "between" two instructions. > > > > I have to confess I haven't really thought the scenario through. My idea > > was to simply add a simple check for an irq context into ->flags setting > > routine because this should never be done in the first place. Not only > > for scope gfp flags but any other PF_ flags IIRC. > > That's not automatically clear to me. There are plenty of places > where an interrupt borrows the context of the task that it happens to > have interrupted. Specifically, interrupts should be using GFP_ATOMIC > anyway, so this doesn't really make a lot of sense, but I don't think > it's necessarily wrong for an interrupt to call a function that says > "Definitely don't make GFP_FS allocations between these two points". Not sure I got your point. IRQ context never does reclaim so anything outside of NOWAIT/ATOMIC is pointless. But you might be refering to a future code where GFP_FS might have a meaning outside of the reclaim context? Anyway if we are to allow modifying PF_ flags from an interrupt contenxt then I believe we should make that code IRQ aware at least. I do not feel really comfortable about async modifications when this is stated to be safe doing in a non atomic way. But I suspect we have drifted away from the original issue. I thought that a simple check would help us narrow down this particular case and somebody messing up from the IRQ context didn't sound like a completely off. -- Michal Hocko SUSE Labs