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=-9.0 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,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 D1909C04AAC for ; Mon, 6 May 2019 07:46:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 94E7E206A3 for ; Mon, 6 May 2019 07:46:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=ffwll.ch header.i=@ffwll.ch header.b="g+Rj6lZO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726404AbfEFHqC (ORCPT ); Mon, 6 May 2019 03:46:02 -0400 Received: from mail-ed1-f65.google.com ([209.85.208.65]:46015 "EHLO mail-ed1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726322AbfEFHqB (ORCPT ); Mon, 6 May 2019 03:46:01 -0400 Received: by mail-ed1-f65.google.com with SMTP id g57so14220879edc.12 for ; Mon, 06 May 2019 00:46:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=04n4bi/0il+bPmWqsIQ2e+x6fvuMJP2MziNsJbLuWN4=; b=g+Rj6lZOq58m6EpuhkaGKAgELBYFqZayO0UVs6XuXjcKuZ9wZW696L0FAvqpvsC3Sz 5LdY6QKVhDTtVGSkx4p7OhAoLS9tQxxHKqNASJ6XRM523im0HjxD4QMpKJJbxL6qls2A COWjAYliGJcM4AvajlOKV2UoIpFu9r95F/qBI= 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:in-reply-to :references:mime-version:content-transfer-encoding; bh=04n4bi/0il+bPmWqsIQ2e+x6fvuMJP2MziNsJbLuWN4=; b=IhZzFIyoOrub0Jcd9EjvTwrCDRWxtDC/eila+1wB+W9Gx2qoVMPfQ4jXoXquHGFNnk 5eMMIPTUWQj6daYk8OIcgViJ7Mv1Q6/9NOiQNUEBMMnMM45FF81zvaiaD9h+DpyluKO3 ZLP33mrrAS95Ydc6tyrnnxAXvdmPTKi7Jx+fbgXH9iVeKWS0/O/rAjBPv7KSSujlZsLd mFHKcCp07AFJSJPJ2mmm0isQlUbXme6VOOYNdycFkI4UeT9bBWNKRNV8/FjDxpuepfyD +yfdQkwXCc/BUZJ2Bgk5oBFMYmdHHjz5moqFy42L2gBy8GZN36WlscR+XMxevfE7q1Wt G3dA== X-Gm-Message-State: APjAAAXP4kXu3EYsPwxdS2DoR37UeME/9RVyJsvJsgNd6bt+OVXqpzNe p7xopgQ0NtqB+NHy/sL2gjruGw== X-Google-Smtp-Source: APXvYqzon9mPPFB9xFLuZ56pH3fApyh6Ek73kjZdssOe6Nvobk9Y3fKvc1cQgXfVibqPEzhGE6bNeA== X-Received: by 2002:a50:8bbb:: with SMTP id m56mr24893171edm.230.1557128759478; Mon, 06 May 2019 00:45:59 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:569e:0:3106:d637:d723:e855]) by smtp.gmail.com with ESMTPSA id z11sm1451872ejo.45.2019.05.06.00.45.58 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 06 May 2019 00:45:58 -0700 (PDT) From: Daniel Vetter To: Intel Graphics Development Cc: Daniel Vetter , Daniel Vetter , Peter Zijlstra , Ingo Molnar , Will Deacon , Petr Mladek , Sergey Senozhatsky , Steven Rostedt , John Ogness , linux-kernel@vger.kernel.org Subject: [PATCH] RFC: console: hack up console_lock more v2 Date: Mon, 6 May 2019 09:45:53 +0200 Message-Id: <20190506074553.21464-1-daniel.vetter@ffwll.ch> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190502141643.21080-1-daniel.vetter@ffwll.ch> References: <20190502141643.21080-1-daniel.vetter@ffwll.ch> 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 console_trylock, called from within printk, can be called from pretty much anywhere. Including try_to_wake_up. Note that this isn't common, usually the box is in pretty bad shape at that point already. But it really doesn't help when then lockdep jumps in and spams the logs, potentially obscuring the real backtrace we're really interested in. One case I've seen (slightly simplified backtrace): Call Trace: console_trylock+0xe/0x60 vprintk_emit+0xf1/0x320 printk+0x4d/0x69 __warn_printk+0x46/0x90 native_smp_send_reschedule+0x2f/0x40 check_preempt_curr+0x81/0xa0 ttwu_do_wakeup+0x14/0x220 try_to_wake_up+0x218/0x5f0 pollwake+0x6f/0x90 credit_entropy_bits+0x204/0x310 add_interrupt_randomness+0x18f/0x210 handle_irq+0x67/0x160 do_IRQ+0x5e/0x130 common_interrupt+0xf/0xf This alone isn't a problem, but the spinlock in the semaphore is also still held while waking up waiters (up() -> __up() -> try_to_wake_up() callchain), which then closes the runqueue vs. semaphore.lock loop, and upsets lockdep, which issues a circular locking splat to dmesg. Worse it upsets developers, since we don't want to spam dmesg with clutter when the machine is dying already. Fix this by creating a prinkt_safe_up() which calls wake_up_process outside of the spinlock. This isn't correct in full generality, but good enough for console_lock: - console_lock doesn't use interruptible or killable or timeout down() calls, hence an up() is the only thing that can wake up a process. Hence the process can't get woken and killed and reaped while we try to wake it up too. - semaphore.c always updates the waiter list while under the spinlock, so there's no other races. Specifically another process that races with a quick console_lock/unlock while we've dropped the spinlock already won't see our own waiter. Also cc'ing John Ogness since perhaps his printk rework fixes this all properly. Signed-off-by: Daniel Vetter Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Will Deacon Cc: Petr Mladek Cc: Sergey Senozhatsky Cc: Steven Rostedt Cc: Daniel Vetter Cc: John Ogness Cc: linux-kernel@vger.kernel.org --- v2: My first attempt at patching down_trylock didn't work (thanks to Petr for pointing out), now try to have a special up() that breaks the cycle. -Daniel --- include/linux/semaphore.h | 1 + kernel/locking/semaphore.c | 31 +++++++++++++++++++++++++++++++ kernel/printk/printk.c | 2 +- 3 files changed, 33 insertions(+), 1 deletion(-) diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h index 11c86fbfeb98..7e839c72809d 100644 --- a/include/linux/semaphore.h +++ b/include/linux/semaphore.h @@ -41,6 +41,7 @@ extern int __must_check down_interruptible(struct semaphore *sem); extern int __must_check down_killable(struct semaphore *sem); extern int __must_check down_trylock(struct semaphore *sem); extern int __must_check down_timeout(struct semaphore *sem, long jiffies); +extern void printk_safe_up(struct semaphore *sem); extern void up(struct semaphore *sem); #endif /* __LINUX_SEMAPHORE_H */ diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c index 561acdd39960..84675993dc59 100644 --- a/kernel/locking/semaphore.c +++ b/kernel/locking/semaphore.c @@ -197,6 +197,37 @@ struct semaphore_waiter { bool up; }; +/** + * printk_safe_up - release the semaphore in console_unlock + * @sem: the semaphore to release + * + * Release the semaphore. Unlike mutexes, up() may be called from any + * context and even by tasks which have never called down(). + * + * NOTE: This is a special version of up() for console_unlock only. It is only + * safe if there are no killable, interruptible or timing out down() calls. + */ +void printk_safe_up(struct semaphore *sem) +{ + unsigned long flags; + struct semaphore_waiter *waiter = NULL; + + raw_spin_lock_irqsave(&sem->lock, flags); + if (likely(list_empty(&sem->wait_list))) { + sem->count++; + } else { + waiter = list_first_entry(&sem->wait_list, + struct semaphore_waiter, list); + list_del(&waiter->list); + waiter->up = true; + } + raw_spin_unlock_irqrestore(&sem->lock, flags); + + if (waiter) + wake_up_process(waiter->task); +} +EXPORT_SYMBOL(printk_safe_up); + /* * Because this function is inlined, the 'state' parameter will be * constant, and thus optimised away by the compiler. Likewise the diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 02ca827b8fac..62303929afda 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -234,7 +234,7 @@ static void __up_console_sem(unsigned long ip) mutex_release(&console_lock_dep_map, 1, ip); printk_safe_enter_irqsave(flags); - up(&console_sem); + printk_safe_up(&console_sem); printk_safe_exit_irqrestore(flags); } #define up_console_sem() __up_console_sem(_RET_IP_) -- 2.20.1