From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755316AbbLaE54 (ORCPT ); Wed, 30 Dec 2015 23:57:56 -0500 Received: from mail-pf0-f176.google.com ([209.85.192.176]:34975 "EHLO mail-pf0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753705AbbLaE5w (ORCPT ); Wed, 30 Dec 2015 23:57:52 -0500 Date: Thu, 31 Dec 2015 13:58:59 +0900 From: Sergey Senozhatsky To: Jan Kara Cc: Andrew Morton , Petr Mladek , KY Sri nivasan , Steven Rostedt , linux-kernel@vger.kernel.org, Sergey Senozhatsky , Sergey Senozhatsky Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing too long Message-ID: <20151231045859.GC479@swordfish> References: <20151210145251.GA540@swordfish> <20151222134730.GD7266@quack.suse.cz> <20151231024459.GA479@swordfish> <20151231031337.GB479@swordfish> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="4Ckj6UjgE2iN1+kY" Content-Disposition: inline In-Reply-To: <20151231031337.GB479@swordfish> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --4Ckj6UjgE2iN1+kY Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On (12/31/15 12:13), Sergey Senozhatsky wrote: [..] > cond_resched() does its job there, of course. well, a user process still can > do a lot of call_console_drivers() calls. may be we can check who is calling > console_unlock() and if we have "!printk_sync && !oops_in_progress" (or just printk_sync > test) AND a user process then return from console_unlock() doing irq_work_queue() > and set PRINTK_PENDING_OUTPUT pending bit, the way vprintk_emit() does it. attached two patches, I ended up having on top of yours. just in case. printk: factor out can_printk_async() console_unlock() can be called directly or indirectly by a user space process, so it can end up doing call_console_drivers() loop, which will hold it from returning back to user-space from a syscall for unpredictable amount of time. Factor out can_printk_async() function, which queues an irq work and sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk). vprintk_emit() already does it, add can_printk_async() call to console_unlock() for !PF_KTHREAD processes. and printk: introduce console_sync_mode console_sync_mode() should be called early in panic() to switch printk from async mode to sync. Otherwise, STOP IPIs can arrive to other CPUs too late and those CPUs will see oops_in_progress being 0 again. -ss --4Ckj6UjgE2iN1+kY Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="0001-printk-introduce-console_sync_mode.patch" >>From c3fc955809adab8f497cdc7581e67e1fa29d6517 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Wed, 30 Dec 2015 20:39:12 +0900 Subject: [PATCH 1/2] printk: introduce console_sync_mode console_sync_mode() should be called early in panic() to switch printk from async mode to sync. Otherwise, STOP IPIs can arrive to other CPUs too late and those CPUs will see oops_in_progress being 0 again. --- include/linux/console.h | 1 + kernel/panic.c | 1 + kernel/printk/printk.c | 5 +++++ 3 files changed, 7 insertions(+) diff --git a/include/linux/console.h b/include/linux/console.h index bd19434..f068985 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -150,6 +150,7 @@ extern int console_trylock(void); extern void console_unlock(void); extern void console_conditional_schedule(void); extern void console_unblank(void); +extern void console_sync_mode(void); extern struct tty_driver *console_device(int *); extern void console_stop(struct console *); extern void console_start(struct console *); diff --git a/kernel/panic.c b/kernel/panic.c index b333380..04c8ff4 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -117,6 +117,7 @@ void panic(const char *fmt, ...) if (old_cpu != PANIC_CPU_INVALID && old_cpu != this_cpu) panic_smp_self_stop(); + console_sync_mode(); console_verbose(); bust_spinlocks(1); va_start(args, fmt); diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index de9d31b..47a70a2 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2501,6 +2501,11 @@ void console_unblank(void) console_unlock(); } +void console_sync_mode(void) +{ + printk_sync = true; +} + /* * Return the console tty driver structure and its associated index */ -- 2.6.4 --4Ckj6UjgE2iN1+kY Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="0002-printk-factor-out-can_printk_async.patch" >>From 92f2c0f2a5ed015caa2757dcfec4407d708f8628 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Thu, 31 Dec 2015 13:39:58 +0900 Subject: [PATCH 2/2] printk: factor out can_printk_async() console_unlock() can be called directly or indirectly by a user space process, so it can end up doing call_console_drivers() loop, which will hold it from returning back to user-space from a syscall for unpredictable amount of time. Factor out can_printk_async() function, which queues an irq work and sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk). vprintk_emit() already does it, add can_printk_async() call to console_unlock() for !PF_KTHREAD processes. --- kernel/printk/printk.c | 42 ++++++++++++++++++++++++++++-------------- 1 file changed, 28 insertions(+), 14 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 47a70a2..7d3a8e1 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -355,6 +355,26 @@ int printk_deferred(const char *fmt, ...) return r; } +static bool can_printk_async(bool sync) +{ + /* + * By default we print message to console asynchronously so that kernel + * doesn't get stalled due to slow serial console. That can lead to + * softlockups, lost interrupts, or userspace timing out under heavy + * printing load. + * + * However we resort to synchronous printing of messages during early + * boot, when oops is in progress, or when synchronous printing was + * explicitely requested by kernel parameter. + */ + if (keventd_up() && !oops_in_progress && !sync) { + __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT); + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); + return true; + } + return false; +} + /* Return log buffer address */ char *log_buf_addr_get(void) { @@ -1889,20 +1909,7 @@ asmlinkage int vprintk_emit(int facility, int level, logbuf_cpu = UINT_MAX; raw_spin_unlock(&logbuf_lock); lockdep_on(); - /* - * By default we print message to console asynchronously so that kernel - * doesn't get stalled due to slow serial console. That can lead to - * softlockups, lost interrupts, or userspace timing out under heavy - * printing load. - * - * However we resort to synchronous printing of messages during early - * boot, when oops is in progress, or when synchronous printing was - * explicitely requested by kernel parameter. - */ - if (keventd_up() && !oops_in_progress && !sync_print) { - __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT); - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); - } else + if (!can_printk_async(sync_print)) sync_print = true; local_irq_restore(flags); @@ -2328,6 +2335,13 @@ void console_unlock(void) return; } + if (!(current->flags & PF_KTHREAD) && + can_printk_async(printk_sync)) { + console_locked = 0; + up_console_sem(); + return; + } + /* * Console drivers are called under logbuf_lock, so * @console_may_schedule should be cleared before; however, we may -- 2.6.4 --4Ckj6UjgE2iN1+kY--