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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 9F7ABC433E0 for ; Fri, 5 Mar 2021 09:10:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6C95664F5F for ; Fri, 5 Mar 2021 09:10:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229494AbhCEJJ4 (ORCPT ); Fri, 5 Mar 2021 04:09:56 -0500 Received: from mx2.suse.de ([195.135.220.15]:43310 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229589AbhCEJJh (ORCPT ); Fri, 5 Mar 2021 04:09:37 -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=1614935373; 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=hthewmbKrwUHIzCeCtJI+RGZwjfy/kQItClydNwNTmE=; b=Aq911/1aHFP8RlL9hyE8m/Hc1Du0lJBetMLwPrILKIMT5aA2QcSDLDfeDYXFsUCT0VIcTd 61dHCdY/INdDMgVGYV4suTpXxRUKrqZJ7hPnISrLa2oeyIKPWZOwhTVh+HQOxQ/iJujIMe LO+6aZOFQ0+pvyrkAakLBUlc4rO1hck= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 8DAD4AF24; Fri, 5 Mar 2021 09:09:33 +0000 (UTC) Date: Fri, 5 Mar 2021 10:09:32 +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: <122e8c5d-60b8-52d9-c6a1-00cd61b2e1b6@oracle.com> <06edda9a-dce9-accd-11a3-97f6d5243ed1@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 Tue 02-03-21 19:59:22, Shakeel Butt wrote: > On Tue, Mar 2, 2021 at 1:19 PM Mike Kravetz wrote: > > > > On 3/2/21 6:29 AM, Michal Hocko wrote: > > > On Tue 02-03-21 06:11:51, Shakeel Butt wrote: > > >> On Tue, Mar 2, 2021 at 1:44 AM Michal Hocko wrote: > > >>> > > >>> On Mon 01-03-21 17:16:29, Mike Kravetz wrote: > > >>>> On 3/1/21 9:23 AM, Michal Hocko wrote: > > >>>>> On Mon 01-03-21 08:39:22, Shakeel Butt wrote: > > >>>>>> On Mon, Mar 1, 2021 at 7:57 AM Michal Hocko wrote: > > >>>>> [...] > > >>>>>>> Then how come this can ever be a problem? in_task() should exclude soft > > >>>>>>> irq context unless I am mistaken. > > >>>>>>> > > >>>>>> > > >>>>>> If I take the following example of syzbot's deadlock scenario then > > >>>>>> CPU1 is the one freeing the hugetlb pages. It is in the process > > >>>>>> context but has disabled softirqs (see __tcp_close()). > > >>>>>> > > >>>>>> CPU0 CPU1 > > >>>>>> ---- ---- > > >>>>>> lock(hugetlb_lock); > > >>>>>> local_irq_disable(); > > >>>>>> lock(slock-AF_INET); > > >>>>>> lock(hugetlb_lock); > > >>>>>> > > >>>>>> lock(slock-AF_INET); > > >>>>>> > > > [...] > > >>> Wouldn't something like this help? It is quite ugly but it would be > > >>> simple enough and backportable while we come up with a more rigorous > > >>> solution. What do you think? > > >>> > > >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c > > >>> index 4bdb58ab14cb..c9a8b39f678d 100644 > > >>> --- a/mm/hugetlb.c > > >>> +++ b/mm/hugetlb.c > > >>> @@ -1495,9 +1495,11 @@ static DECLARE_WORK(free_hpage_work, free_hpage_workfn); > > >>> void free_huge_page(struct page *page) > > >>> { > > >>> /* > > >>> - * Defer freeing if in non-task context to avoid hugetlb_lock deadlock. > > >>> + * Defer freeing if in non-task context or when put_page is called > > >>> + * with IRQ disabled (e.g from via TCP slock dependency chain) to > > >>> + * avoid hugetlb_lock deadlock. > > >>> */ > > >>> - if (!in_task()) { > > >>> + if (!in_task() || irqs_disabled()) { > > >> > > >> Does irqs_disabled() also check softirqs? > > > > > > Nope it doesn't AFAICS. I was referring to the above lockdep splat which > > > claims irq disabled to be the trigger. But now that you are mentioning > > > that it would be better to replace in_task() along the way. We have > > > discussed that in another email thread and I was suggesting to use > > > in_atomic() which should catch also bh disabled situation. The big IF is > > > that this needs preempt count to be enabled unconditionally. There are > > > changes in the RCU tree heading that direction. > > > > I have not been following developments in preemption and the RCU tree. > > The comment for in_atomic() says: > > > > /* > > * Are we running in atomic context? WARNING: this macro cannot > > * always detect atomic context; in particular, it cannot know about > > * held spinlocks in non-preemptible kernels. Thus it should not be > > * used in the general case to determine whether sleeping is possible. > > * Do not use in_atomic() in driver code. > > */ > > > > That does seem to be the case. I verified in_atomic can detect softirq > > context even in non-preemptible kernels. But, as the comment says it > > will not detect a held spinlock in non-preemptible kernels. So, I think > > in_atomic would be better than the current check for !in_task. That > > would handle this syzbot issue, but we could still have issues if the > > hugetlb put_page path is called while someone is holding a spinlock with > > all interrupts enabled. Looks like there is no way to detect this > > today in non-preemptible kernels. in_atomic does detect spinlocks held > > in preemptible kernels. > > > > I might suggest changing !in_task to in_atomic for now, and then work on > > a more robust solution. I'm afraid such a robust solution will > > require considerable effort. It would need to handle put_page being > > called in any context: hardirq, softirq, spinlock held ... The > > put_page/free_huge_page path will need to offload (workqueue or > > something else) any processing that can possibly sleep. > > > > Is it worth making the in_atomic change now, or should we just start > > working on the more robust complete solution? > > IMHO the change to in_atomic is beneficial because it will at least > fix this specific issue. No reason to keep the users of TCP TX > zerocopy from hugetlb pages broken for a more comprehensive solution. Another option would be to select PREEMPT_COUNT when hugetlb is enabled. That would reduce dependency on a patch I was talking about in other email. Not nice but here we are... -- Michal Hocko SUSE Labs