Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752603Ab2JBN2N (ORCPT ); Tue, 2 Oct 2012 09:28:13 -0400 Received: from mail-pb0-f46.google.com ([209.85.160.46]:45681 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751566Ab2JBN2K (ORCPT ); Tue, 2 Oct 2012 09:28:10 -0400 Date: Tue, 2 Oct 2012 06:28:05 -0700 From: Greg KH To: Daniel Vetter Cc: LKML , Peter Zijlstra , Intel Graphics Development , DRI Development , Thomas Gleixner , Alan Cox Subject: Re: [Intel-gfx] [PATCH] console: implement lockdep support for console_lock Message-ID: <20121002132805.GA30187@kroah.com> References: <87627b9453.fsf@intel.com> <1348336331-20957-1-git-send-email-daniel.vetter@ffwll.ch> <20120922200629.GC14004@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5791 Lines: 128 On Tue, Oct 02, 2012 at 02:56:48PM +0200, Daniel Vetter wrote: > On Sat, Sep 22, 2012 at 10:06 PM, Greg KH wrote: > > On Sat, Sep 22, 2012 at 07:52:11PM +0200, Daniel Vetter wrote: > >> Dave Airlie recently discovered a locking bug in the fbcon layer, > >> where a timer_del_sync (for the blinking cursor) deadlocks with the > >> timer itself, since both (want to) hold the console_lock: > >> > >> https://lkml.org/lkml/2012/8/21/36 > >> > >> Unfortunately the console_lock isn't a plain mutex and hence has no > >> lockdep support. Which resulted in a few days wasted of tracking down > >> this bug (complicated by the fact that printk doesn't show anything > >> when the console is locked) instead of noticing the bug much earlier > >> with the lockdep splat. > >> > >> Hence I've figured I need to fix that for the next deadlock involving > >> console_lock - and with kms/drm growing ever more complex locking > >> that'll eventually happen. > >> > >> Now the console_lock has rather funky semantics, so after a quick irc > >> discussion with Thomas Gleixner and Dave Airlie I've quickly ditched > >> the original idead of switching to a real mutex (since it won't work) > >> and instead opted to annotate the console_lock with lockdep > >> information manually. > >> > >> There are a few special cases: > >> - The console_lock state is protected by the console_sem, and usually > >> grabbed/dropped at _lock/_unlock time. But the suspend/resume code > >> drops the semaphore without dropping the console_lock (see > >> suspend_console/resume_console). But since the same thread that did > >> the suspend will do the resume, we don't need to fix up anything. > >> > >> - In the printk code there's a special trylock, only used to kick off > >> the logbuffer printk'ing in console_unlock. But all that happens > >> while lockdep is disable (since printk does a few other evil > >> tricks). So no issue there, either. > >> > >> - The console_lock can also be acquired form irq context (but only > >> with a trylock). lockdep already handles that. > >> > >> This all leaves us with annotating the normal console_lock, _unlock > >> and _trylock functions. > >> > >> And yes, it works - simply unloading a drm kms driver resulted in > >> lockdep complaining about the deadlock in fbcon_deinit: > >> > >> ====================================================== > >> [ INFO: possible circular locking dependency detected ] > >> 3.6.0-rc2+ #552 Not tainted > >> ------------------------------------------------------- > >> kms-reload/3577 is trying to acquire lock: > >> ((&info->queue)){+.+...}, at: [] wait_on_work+0x0/0xa7 > >> > >> but task is already holding lock: > >> (console_lock){+.+.+.}, at: [] bind_con_driver+0x38/0x263 > >> > >> which lock already depends on the new lock. > >> > >> the existing dependency chain (in reverse order) is: > >> > >> -> #1 (console_lock){+.+.+.}: > >> [] lock_acquire+0x95/0x105 > >> [] console_lock+0x59/0x5b > >> [] fb_flashcursor+0x2e/0x12c > >> [] process_one_work+0x1d9/0x3b4 > >> [] worker_thread+0x1a7/0x24b > >> [] kthread+0x7f/0x87 > >> [] kernel_thread_helper+0x4/0x10 > >> > >> -> #0 ((&info->queue)){+.+...}: > >> [] __lock_acquire+0x999/0xcf6 > >> [] lock_acquire+0x95/0x105 > >> [] wait_on_work+0x3b/0xa7 > >> [] __cancel_work_timer+0xbf/0x102 > >> [] cancel_work_sync+0xb/0xd > >> [] fbcon_deinit+0x11c/0x1dc > >> [] bind_con_driver+0x145/0x263 > >> [] unbind_con_driver+0x14f/0x195 > >> [] store_bind+0x1ad/0x1c1 > >> [] dev_attr_store+0x13/0x1f > >> [] sysfs_write_file+0xe9/0x121 > >> [] vfs_write+0x9b/0xfd > >> [] sys_write+0x3e/0x6b > >> [] system_call_fastpath+0x16/0x1b > >> > >> other info that might help us debug this: > >> > >> Possible unsafe locking scenario: > >> > >> CPU0 CPU1 > >> ---- ---- > >> lock(console_lock); > >> lock((&info->queue)); > >> lock(console_lock); > >> lock((&info->queue)); > >> > >> *** DEADLOCK *** > >> > >> v2: Mark the lockdep_map static, noticed by Jani Nikula. > >> > >> Cc: Dave Airlie > >> Cc: Thomas Gleixner > >> Cc: Alan Cox > >> Cc: Peter Zijlstra > >> Signed-off-by: Daniel Vetter > >> --- > >> kernel/printk.c | 9 +++++++++ > >> 1 file changed, 9 insertions(+) > > > > So I'm guessing I should take this through the tty tree, right? Any > > objections to that for 3.7? > > I've noticed that the tty tree went in already :( Any chance you could > still slip this in for 3.7? I'd _really_ like to have this stuff in > for debugging console_lock madness in drm drivers - we've already had > our fair share of those ... No, as it hasn't been in linux-next already, I can't send it in for 3.7, sorry, you know that. I'll be glad to queue it up for 3.8 if you want me to. thanks, greg k-h -- 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/