From patchwork Fri Mar 24 16:14:05 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Petr Mladek X-Patchwork-Id: 9643215 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 2995C60328 for ; Fri, 24 Mar 2017 16:15:23 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 1B8A41FF22 for ; Fri, 24 Mar 2017 16:15:23 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 106C420223; Fri, 24 Mar 2017 16:15:23 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 6EEAF2521E for ; Fri, 24 Mar 2017 16:15:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966244AbdCXQPS (ORCPT ); Fri, 24 Mar 2017 12:15:18 -0400 Received: from mx2.suse.de ([195.135.220.15]:51321 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966143AbdCXQOk (ORCPT ); Fri, 24 Mar 2017 12:14:40 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 8BED7ABB1; Fri, 24 Mar 2017 16:14:32 +0000 (UTC) From: Petr Mladek To: Sergey Senozhatsky Cc: Tetsuo Handa , Steven Rostedt , Peter Zijlstra , Andrew Morton , Greg Kroah-Hartman , Jiri Slaby , linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Petr Mladek Subject: [PATCH v3] printk: Correctly handle preemption in console_unlock() Date: Fri, 24 Mar 2017 17:14:05 +0100 Message-Id: <1490372045-22288-1-git-send-email-pmladek@suse.com> X-Mailer: git-send-email 1.8.5.6 In-Reply-To: <20170202093011.1e067ea7@gandalf.local.home> References: <20170202093011.1e067ea7@gandalf.local.home> Sender: linux-fbdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fbdev@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Some console drivers code calls console_conditional_schedule() that looks at @console_may_schedule. The value must be cleared when the drivers are called from console_unlock() with interrupts disabled. But rescheduling is fine when the same code is called, for example, from tty operations where the console semaphore is taken via console_lock(). This is why @console_may_schedule is cleared before calling console drivers. The original value is stored to decide if we could sleep between lines. Now, @console_may_schedule is not cleared when we call console_trylock() and jump back to the "again" goto label. This has become a problem, since the commit 6b97a20d3a7909daa066 ("printk: set may_schedule for some of console_trylock() callers"). @console_may_schedule might get enabled now. There is also the opposite problem. console_lock() can be called only from preemptive context. It can always enable scheduling in the console code. But console_trylock() is not able to detect it when CONFIG_PREEMPT_COUNT is disabled. Therefore we should use the original @console_may_schedule value after re-acquiring the console semaphore in console_unlock(). This patch solves both problems by moving the "again" goto label. Alternative solution was to clear and restore the value around call_console_drivers(). Then console_conditional_schedule() could be used also inside console_unlock(). But there was a potential race with console_flush_on_panic() as reported by Sergey Senozhatsky. That function should be called only where there is only one CPU and with interrupts disabled. But better be on the safe side because stopping CPUs might fail. Fixes: 6b97a20d3a7909 ("printk: set may_schedule for some of console_trylock() callers") Suggested-by: Tetsuo Handa Signed-off-by: Petr Mladek Reviewed-by: Sergey Senozhatsky --- Link to v2: https://lkml.kernel.org/r/1485353325-26591-1-git-send-email-pmladek@suse.com Changes against v3: + do not remove useful details from the original comment as suggested by Steven and Sergey + fix typo in the new comment Changes against v2: + use conservative solution with the following rules: + always clear console_may_schedule after again goto label + save and use the original value to decide about sleeping inside console unlock + do not set console_may_schedule using the saved value; it avoids potential race with console_flush_on_panic(); also it avoids breaking the complex logic used in other functions manipulating this variable. kernel/printk/printk.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) 1.8.5.6 -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 2984fb0f0257..e5636fa04e66 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2161,7 +2161,7 @@ void console_unlock(void) } /* - * Console drivers are called under logbuf_lock, so + * Console drivers are called with interrupts disabled, so * @console_may_schedule should be cleared before; however, we may * end up dumping a lot of lines, for example, if called from * console registration path, and should invoke cond_resched() @@ -2169,12 +2169,15 @@ void console_unlock(void) * scheduling stall on a slow console leading to RCU stall and * softlockup warnings which exacerbate the issue with more * messages practically incapacitating the system. + * + * console_trylock() is not able to detect the preemptive + * context reliably. Therefore the value must be stored before + * and cleared after the the "again" goto label. */ do_cond_resched = console_may_schedule; +again: console_may_schedule = 0; -again: /* * We released the console_sem lock, so we need to recheck if * cpu is online and (if not) is there at least one CON_ANYTIME --