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 0A0ABC433E0 for ; Mon, 1 Mar 2021 15:58:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A3AC764D8F for ; Mon, 1 Mar 2021 15:58:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237483AbhCAP5u (ORCPT ); Mon, 1 Mar 2021 10:57:50 -0500 Received: from mx2.suse.de ([195.135.220.15]:33604 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237445AbhCAP5t (ORCPT ); Mon, 1 Mar 2021 10:57:49 -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=1614614222; 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=jEMLd+8QwG0qM6Ywh0J3rChAMkV49Kqjdwq8Zo3TTnc=; b=YWq5yRLYt45aCcw4QbD8nB6AbXIPl6KxmjoPhKZDaMcdVnvqBxwDAQgddKSz/LD5VqWYvi MICsxh1rObrONr37HbMxKUjrsC4BzAWovbHGGdIkbXY5y2H36PKpA6RMda8D35iD+AszTs N3lSjtmaOyTCu08PpwblQwl+37TTT1k= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 91E3AAFFB; Mon, 1 Mar 2021 15:57:02 +0000 (UTC) Date: Mon, 1 Mar 2021 16:57:01 +0100 From: Michal Hocko To: Shakeel Butt Cc: Mike Kravetz , syzbot , Andrew Morton , LKML , Linux MM , syzkaller-bugs , Eric Dumazet , Mina Almasry Subject: Re: possible deadlock in sk_clone_lock Message-ID: References: <000000000000f1c03b05bc43aadc@google.com> <7b7c4f41-b72e-840f-278a-320b9d97f887@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 01-03-21 07:10:11, Shakeel Butt wrote: > On Mon, Mar 1, 2021 at 4:12 AM Michal Hocko wrote: > > > > On Fri 26-02-21 16:00:30, Shakeel Butt wrote: > > > On Fri, Feb 26, 2021 at 3:14 PM Mike Kravetz wrote: > > > > > > > > Cc: Michal > > > > > > > > On 2/26/21 2:44 PM, Shakeel Butt wrote: > > > > > On Fri, Feb 26, 2021 at 2:09 PM syzbot > > > > > wrote: > > > > > > > > >> other info that might help us debug this: > > > > >> > > > > >> Possible interrupt unsafe locking scenario: > > > > >> > > > > >> CPU0 CPU1 > > > > >> ---- ---- > > > > >> lock(hugetlb_lock); > > > > >> local_irq_disable(); > > > > >> lock(slock-AF_INET); > > > > >> lock(hugetlb_lock); > > > > >> > > > > >> lock(slock-AF_INET); > > > > >> > > > > >> *** DEADLOCK *** > > > > > > > > > > This has been reproduced on 4.19 stable kernel as well [1] and there > > > > > is a reproducer as well. > > > > > > > > > > It seems like sendmsg(MSG_ZEROCOPY) from a buffer backed by hugetlb. I > > > > > wonder if we just need to make hugetlb_lock softirq-safe. > > > > > > > > > > [1] https://syzkaller.appspot.com/bug?extid=6383ce4b0b8ec575ad93 > > > > > > > > Thanks Shakeel, > > > > > > > > Commit c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in non-task > > > > context") attempted to address this issue. It uses a work queue to > > > > acquire hugetlb_lock if the caller is !in_task(). > > > > > > > > In another recent thread, there was the suggestion to change the > > > > !in_task to in_atomic. > > > > > > > > I need to do some research on the subtle differences between in_task, > > > > in_atomic, etc. TBH, I 'thought' !in_task would prevent the issue > > > > reported here. But, that obviously is not the case. > > > > > > I think the freeing is happening in the process context in this report > > > but it is creating the lock chain from softirq-safe slock to > > > irq-unsafe hugetlb_lock. So, two solutions I can think of are: (1) > > > always defer the freeing of hugetlb pages to a work queue or (2) make > > > hugetlb_lock softirq-safe. > > > > There is __do_softirq so this should be in the soft IRQ context no? > > Is this really reproducible with kernels which have c77c0a8ac4c5 > > applied? > > Yes this is softirq context and syzbot has reproduced this on > linux-next 20210224. Then how come this can ever be a problem? in_task() should exclude soft irq context unless I am mistaken. > > Btw. making hugetlb lock irq safe has been already discussed and it > > seems to be much harder than expected as some heavy operations are done > > under the lock. This is really bad. > > What about just softirq-safe i.e. spin_[un]lock_bh()? Will it still be that bad? This would be a similar problem to the irq variant. It would just result in soft irq being delayed potentially. > > Postponing the whole freeing > > operation into a worker context is certainly possible but I would > > consider it rather unfortunate. We would have to add some sync mechanism > > to wait for hugetlb pages in flight to prevent from external > > observability to the userspace. E.g. when shrinking the pool. > > I think in practice recycling of hugetlb pages is a rare event, so we > might get away without the sync mechanism. How about start postponing > the freeing without sync mechanism and add it later if there are any > user reports complaining? I think this should be a last resort. Maybe we can come up with something better. E.g. break down the hugetlb_lock and use something different for expensive operations. -- Michal Hocko SUSE Labs