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=-9.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT 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 CFC57C35280 for ; Thu, 7 May 2020 22:54:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AAA94208CA for ; Thu, 7 May 2020 22:54:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="WAPAb6AJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727124AbgEGWyV (ORCPT ); Thu, 7 May 2020 18:54:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43226 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726515AbgEGWyV (ORCPT ); Thu, 7 May 2020 18:54:21 -0400 Received: from mail-pj1-x1043.google.com (mail-pj1-x1043.google.com [IPv6:2607:f8b0:4864:20::1043]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D49FFC05BD43 for ; Thu, 7 May 2020 15:54:20 -0700 (PDT) Received: by mail-pj1-x1043.google.com with SMTP id q24so3316005pjd.1 for ; Thu, 07 May 2020 15:54:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=X1eOoZGo2RhELWP8ZXLrEQsjy79crnPXYewegVbC2bc=; b=WAPAb6AJ5skxQqiGjsj4YLprJto6hJMDsC1OXq2025SYCXQw680iFz8cZQDTQ1DckJ EmxNFgDSkka2NqoQTFhhqT/tuSIZd2brTu/CJYiVc8+eMuFmzWsQT+pvUbKme/7dYNq5 oV1FD3X4i27cGhlx9grXyhUdkRWJpiSoqqf6w= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=X1eOoZGo2RhELWP8ZXLrEQsjy79crnPXYewegVbC2bc=; b=BMcvs8Wb6kVEyfZ3A9cz77LSIg8lKNyraK1R6tI2Bng4W01Q93AhI1vLwgWhwnqcZo 8MZZEtHfjXG/euOagjBmsRYRh/SKTGhxun/JV+MG4gJNMCVaQey+f3KpxHS4UzoMXkj+ qsDSfxWvAjL6AROEhykpDRqyiXfr5Nk1m15olmRISRgihnBu/9K5cF7UOmK6IvkrmH+x sFHk0O7FaGUSUKQmkKDnQHiR7GJ1zg9x3zkpgeMWxI9/ZxCof4lXms5ydsb1rdtjJmwp 60K/0bH2ZFP8NhksiO6A8Km/bAHL3lTvlAr4Pp8dqWfNywHU2Ar+A9r5uDi75u5RHpLH J0mg== X-Gm-Message-State: AGi0PuYxud6twgHjRxUy+d/VP0vnQ4bj/yurY2r4pDLjrsaIhlDeg617 avxIzOJ0iOgLMcFZaNLd1lJvew== X-Google-Smtp-Source: APiQypJKFKpsytswpG/Q3vS6VvEccA4XHAZJIeSSZXlao5nBRjVP6Zd0WcIrMxJfYOyASvW1D28v3w== X-Received: by 2002:a17:90a:ba88:: with SMTP id t8mr2525688pjr.182.1588892059212; Thu, 07 May 2020 15:54:19 -0700 (PDT) Received: from tictac2.mtv.corp.google.com ([2620:15c:202:1:24fa:e766:52c9:e3b2]) by smtp.gmail.com with ESMTPSA id k6sm763768pju.44.2020.05.07.15.54.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 07 May 2020 15:54:18 -0700 (PDT) From: Douglas Anderson To: Jason Wessel , Daniel Thompson Cc: sumit.garg@linaro.org, Douglas Anderson , kgdb-bugreport@lists.sourceforge.net, linux-kernel@vger.kernel.org Subject: [PATCH] kgdb: Avoid suspicious RCU usage warning Date: Thu, 7 May 2020 15:53:58 -0700 Message-Id: <20200507153444.1.I70e0d4fd46d5ed2aaf0c98a355e8e1b7a5bb7e4e@changeid> X-Mailer: git-send-email 2.26.2.645.ge9eca65c58-goog MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org At times when I'm using kgdb I see a splat on my console about suspicious RCU usage. I managed to come up with a case that could reproduce this that looked like this: WARNING: suspicious RCU usage 5.7.0-rc4+ #609 Not tainted ----------------------------- kernel/pid.c:395 find_task_by_pid_ns() needs rcu_read_lock() protection! other info that might help us debug this: rcu_scheduler_active = 2, debug_locks = 1 3 locks held by swapper/0/1: #0: ffffff81b6b8e988 (&dev->mutex){....}-{3:3}, at: __device_attach+0x40/0x13c #1: ffffffd01109e9e8 (dbg_master_lock){....}-{2:2}, at: kgdb_cpu_enter+0x20c/0x7ac #2: ffffffd01109ea90 (dbg_slave_lock){....}-{2:2}, at: kgdb_cpu_enter+0x3ec/0x7ac stack backtrace: CPU: 7 PID: 1 Comm: swapper/0 Not tainted 5.7.0-rc4+ #609 Hardware name: Google Cheza (rev3+) (DT) Call trace: dump_backtrace+0x0/0x1b8 show_stack+0x1c/0x24 dump_stack+0xd4/0x134 lockdep_rcu_suspicious+0xf0/0x100 find_task_by_pid_ns+0x5c/0x80 getthread+0x8c/0xb0 gdb_serial_stub+0x9d4/0xd04 kgdb_cpu_enter+0x284/0x7ac kgdb_handle_exception+0x174/0x20c kgdb_brk_fn+0x24/0x30 call_break_hook+0x6c/0x7c brk_handler+0x20/0x5c do_debug_exception+0x1c8/0x22c el1_sync_handler+0x3c/0xe4 el1_sync+0x7c/0x100 rpmh_rsc_probe+0x38/0x420 platform_drv_probe+0x94/0xb4 really_probe+0x134/0x300 driver_probe_device+0x68/0x100 __device_attach_driver+0x90/0xa8 bus_for_each_drv+0x84/0xcc __device_attach+0xb4/0x13c device_initial_probe+0x18/0x20 bus_probe_device+0x38/0x98 device_add+0x38c/0x420 If I understand properly we should just be able to blanket kgdb under one big RCU read lock and the problem should go away. We'll add it to the beast-of-a-function known as kgdb_cpu_enter(). With this I no longer get any splats and things seem to work fine. Signed-off-by: Douglas Anderson --- kernel/debug/debug_core.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c index 2b7c9b67931d..5155cf06731b 100644 --- a/kernel/debug/debug_core.c +++ b/kernel/debug/debug_core.c @@ -564,6 +564,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, int online_cpus = num_online_cpus(); u64 time_left; + rcu_read_lock(); + kgdb_info[ks->cpu].enter_kgdb++; kgdb_info[ks->cpu].exception_state |= exception_state; @@ -635,6 +637,7 @@ 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); + rcu_read_unlock(); return 0; } cpu_relax(); @@ -773,6 +776,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock(); + return kgdb_info[cpu].ret_state; } -- 2.26.2.645.ge9eca65c58-goog