Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759481AbYARBF3 (ORCPT ); Thu, 17 Jan 2008 20:05:29 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751634AbYARBFT (ORCPT ); Thu, 17 Jan 2008 20:05:19 -0500 Received: from ms-smtp-03.nyroc.rr.com ([24.24.2.57]:52011 "EHLO ms-smtp-03.nyroc.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757242AbYARBFS (ORCPT ); Thu, 17 Jan 2008 20:05:18 -0500 Date: Thu, 17 Jan 2008 20:04:27 -0500 (EST) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: LKML cc: Linus Torvalds , Andrew Morton , Alexey Dobriyan , Kirill Korotaev Subject: [PATCH] printk deadlocks if called with runqueue lock held Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2238 Lines: 57 I thought that one could place a printk anywhere without worrying. But it seems that it is not wise to place a printk where the runqueue lock is held. I just spent two hours debugging why some of my code was locking up, to find that the lockup was caused by some debugging printk's that I had in the scheduler. The printk's were only in rare paths so they shouldn't be too much of a problem, but after I hit the printk the system locked up. Thinking that it was locking up on my code I went looking down the wrong path. I finally found (after examining an NMI dump) that the lockup happened because printk was trying to wakeup the klogd daemon, which caused a deadlock when the try_to_wakeup code tries to grab the runqueue lock. Since printks are seldom called with interrupts disabled, we can hold off the waking of klogd if they are. We don't have access to the runqueue locks from printk, but those locks need interrupts disabled in order to be held. Calling printk with interrupts disabled should only be done for emergencies and debugging anyway. And with this patch, my code ran fine ;-) Signed-off-by: Steven Rostedt --- kernel/printk.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) Index: linux-mcount.git/kernel/printk.c =================================================================== --- linux-mcount.git.orig/kernel/printk.c 2008-01-17 09:06:23.000000000 -0500 +++ linux-mcount.git/kernel/printk.c 2008-01-17 19:56:59.000000000 -0500 @@ -978,7 +978,13 @@ void release_console_sem(void) console_locked = 0; up(&console_sem); spin_unlock_irqrestore(&logbuf_lock, flags); - if (wake_klogd) + /* + * If we try to wake up klogd while printing with the runqueue lock + * held, this will deadlock. We don't have access to the runqueue + * lock from here, but just checking for interrupts disabled + * should be enough. + */ + if (!irqs_disabled() && wake_klogd) wake_up_klogd(); } EXPORT_SYMBOL(release_console_sem); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/