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=-6.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 79B46CA90AF for ; Wed, 13 May 2020 13:35:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 47EF820643 for ; Wed, 13 May 2020 13:35:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linaro.org header.i=@linaro.org header.b="Z3zy8N5e" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388549AbgEMNfQ (ORCPT ); Wed, 13 May 2020 09:35:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56176 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388498AbgEMNfC (ORCPT ); Wed, 13 May 2020 09:35:02 -0400 Received: from mail-lf1-x142.google.com (mail-lf1-x142.google.com [IPv6:2a00:1450:4864:20::142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 78C50C061A0C for ; Wed, 13 May 2020 06:35:02 -0700 (PDT) Received: by mail-lf1-x142.google.com with SMTP id r17so10608123lff.9 for ; Wed, 13 May 2020 06:35:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=AbLaM0zuS8RSNZ5KCHHZ6Ac152Q0ScrgI1ljQOu9aJE=; b=Z3zy8N5e/5EgnatAgskbT9OXirHcNqQ7fTlo/YKqHV1m5BM1RVkKBpuiIyRaEQWi7A zO6YxV0ROpQ6WIuWP9zXO7fqir2p3ilOa6tRn/Sk3FLWOMVdmfVet/OBGYLotV2ovWuF Hrds8StM+QFbwfMZbrm1xouLulRg/WzYRi305dZ8QDIuF/vaH+uB/GZm6iSOVPqxl/g3 Xlxwd39klDGBj45LEzSyGeKLdICjpN+To1D5o2xAS1FchGZ5tPXjAikGdum/gp+y23Bh llqYknMfCUZRZGmHkN0z7/eyytGU+4k9LSObmyuXIWY/uqRHd9fMpbBZn5eap5HGsqxn 9wdw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=AbLaM0zuS8RSNZ5KCHHZ6Ac152Q0ScrgI1ljQOu9aJE=; b=C3Ys1FVyQZ7FCwOrba2pUYR7vqDAnYDjJFXwTgNX4e4FUjy7vYvgJZU5BuOZdEk2hb K97NkWpsLYpkNMDqApIg13XO7vrLWcwwnp1vdBuY9rd4Krs/5NUejQBjnOCz9HzVtY39 TBtOGrBtS/jUVncTicUG3QD5c54VG6rTd4xrpxbfOEq9VxJQEf3p17oiTNN23QFJVaN/ iQS8sVe/SLFLLy/i/Si8m3dSN7M+4ivKWTB07HmT+2amtgJKgT/rvhzUzfX10k13qeXJ I0Fw02z+Im7yElGMM5Q9SRLTtUpjYQBW6HhxgucZZK/+nI7pgo3aWTK+ERX4rET/OQtV fdLw== X-Gm-Message-State: AOAM532MQPxjBtBxi85nIBgg7I1lduoQRJbNUzTq/E8aoH9b2I2yQk+a /+yLXgZC01Y5mKMDKnP8HirtXjhhi1PjGJmxciPX3A== X-Google-Smtp-Source: ABdhPJwfDPLhZmMoBn1O6IHENozAHHXWYdX6zwDLq85eaDYbcafk51r8oY0LcH3KbleB+lci1jFw6H8KtWuXpRg1hpU= X-Received: by 2002:ac2:4293:: with SMTP id m19mr8978614lfh.204.1589376899941; Wed, 13 May 2020 06:34:59 -0700 (PDT) MIME-Version: 1.0 References: <1589273314-12060-1-git-send-email-sumit.garg@linaro.org> <20200512142533.ta4uejwmq5gchtlx@holly.lan> In-Reply-To: <20200512142533.ta4uejwmq5gchtlx@holly.lan> From: Sumit Garg Date: Wed, 13 May 2020 19:04:48 +0530 Message-ID: Subject: Re: [PATCH] kgdb: Fix broken handling of printk() in NMI context To: Daniel Thompson Cc: Jason Wessel , Douglas Anderson , kgdb-bugreport@lists.sourceforge.net, Linux Kernel Mailing List , Arnd Bergmann , Andrew Morton , Petr Mladek , Peter Zijlstra , Steven Rostedt Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 12 May 2020 at 19:55, Daniel Thompson wrote: > > On Tue, May 12, 2020 at 02:18:34PM +0530, Sumit Garg wrote: > > Since commit 42a0bb3f7138 ("printk/nmi: generic solution for safe printk > > in NMI"), kgdb entry in NMI context defaults to use safe NMI printk() > > I didn't see the author on Cc: nor any of the folks whose hands it > passed through. It would definitely be good to involve them in this > discussion. > Thanks for updating the Cc: list. > > > which involves CPU specific buffers and deferred printk() until exit from > > NMI context. > > > > But kgdb being a stop-the-world debugger, we don't want to defer printk() > > especially backtrace on corresponding CPUs. So instead switch to normal > > printk() mode in kgdb_cpu_enter() if entry is in NMI context. > > So, firstly I should *definitely* take a mea cupla for not shouting > about this at the time (I was on Cc:... twice). Only thing I can say > confidently is that the test suite didn't yell about this and so I > didn't look at this as closely as I should have done (and that it > didn't yell is mostly because I'm still building out the test suite > coverage). > > Anyhow... > > This feels a little like we are smearing the printk() interception logic > across the kernel in ways that make things hard to read. If we accepted > this patch we then have, the new NMI interception logic, the old kdb > interception logic and some hacks in the kgdb trap handler to defang the > NMI interception logic and force the kdb logic to kick in. > > Wouldn't it be better to migrate kdb interception logic up a couple of > levels so that it continues to function even when we are in nmi printk > mode. That way *all* the printk() interception code would end up in > one place. > Yes it would be better to have all printk() interception code at one place. Let me see if I can come up with an integrated logic. > Finally some clue description of how to provoke the problem would be > useful... that sort of things helps me to grow the test suite coverage. > Sure I will update the description. BTW, this issue can be easily reproduced via issuing a backtrace (kdb command: "bt or btc") on a CPU which entered kgdb in NMI context. -Sumit > > Daniel. > > > > > > Signed-off-by: Sumit Garg > > --- > > > > Similar change was posted earlier specific to arm64 here [1]. But after > > discussions it emerged out that this broken handling of printk() in NMI > > context should be a common problem that is relevant to other archs as well. > > So fix this handling in kgdb_cpu_enter() as there can be multiple entry > > points to kgdb in NMI context. > > > > [1] https://lkml.org/lkml/2020/4/24/328 > > > > kernel/debug/debug_core.c | 13 +++++++++++++ > > 1 file changed, 13 insertions(+) > > > > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c > > index 2b7c9b6..ab2933f 100644 > > --- a/kernel/debug/debug_core.c > > +++ b/kernel/debug/debug_core.c > > @@ -567,6 +567,15 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, > > kgdb_info[ks->cpu].enter_kgdb++; > > kgdb_info[ks->cpu].exception_state |= exception_state; > > > > + /* > > + * kgdb entry in NMI context defaults to use safe NMI printk() which > > + * involves CPU specific buffers and deferred printk() until exit from > > + * NMI context. But kgdb being a stop-the-world debugger, we don't want > > + * to defer printk(). So instead switch to normal printk() mode here. > > + */ > > + if (in_nmi()) > > + printk_nmi_exit(); > > + > > if (exception_state == DCPU_WANT_MASTER) > > atomic_inc(&masters_in_kgdb); > > else > > @@ -635,6 +644,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, > > atomic_dec(&slaves_in_kgdb); > > dbg_touch_watchdogs(); > > local_irq_restore(flags); > > + if (in_nmi()) > > + printk_nmi_enter(); > > return 0; > > } > > cpu_relax(); > > @@ -772,6 +783,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, > > raw_spin_unlock(&dbg_master_lock); > > dbg_touch_watchdogs(); > > local_irq_restore(flags); > > + if (in_nmi()) > > + printk_nmi_enter(); > > > > return kgdb_info[cpu].ret_state; > > } > > -- > > 2.7.4 > >