Re: [Intel-gfx] [PATCH] console: implement lockdep support for console_lock

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Oct 02, 2012 at 02:56:48PM +0200, Daniel Vetter wrote:
> On Sat, Sep 22, 2012 at 10:06 PM, Greg KH <gregkh@linuxfoundation.org> 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: [<ffffffff81058c70>] wait_on_work+0x0/0xa7
> >>
> >> but task is already holding lock:
> >>  (console_lock){+.+.+.}, at: [<ffffffff81264686>] bind_con_driver+0x38/0x263
> >>
> >> which lock already depends on the new lock.
> >>
> >> the existing dependency chain (in reverse order) is:
> >>
> >> -> #1 (console_lock){+.+.+.}:
> >>        [<ffffffff81087440>] lock_acquire+0x95/0x105
> >>        [<ffffffff81040190>] console_lock+0x59/0x5b
> >>        [<ffffffff81209cb6>] fb_flashcursor+0x2e/0x12c
> >>        [<ffffffff81057c3e>] process_one_work+0x1d9/0x3b4
> >>        [<ffffffff810584a2>] worker_thread+0x1a7/0x24b
> >>        [<ffffffff8105ca29>] kthread+0x7f/0x87
> >>        [<ffffffff813b1204>] kernel_thread_helper+0x4/0x10
> >>
> >> -> #0 ((&info->queue)){+.+...}:
> >>        [<ffffffff81086cb3>] __lock_acquire+0x999/0xcf6
> >>        [<ffffffff81087440>] lock_acquire+0x95/0x105
> >>        [<ffffffff81058cab>] wait_on_work+0x3b/0xa7
> >>        [<ffffffff81058dd6>] __cancel_work_timer+0xbf/0x102
> >>        [<ffffffff81058e33>] cancel_work_sync+0xb/0xd
> >>        [<ffffffff8120a3b3>] fbcon_deinit+0x11c/0x1dc
> >>        [<ffffffff81264793>] bind_con_driver+0x145/0x263
> >>        [<ffffffff81264a45>] unbind_con_driver+0x14f/0x195
> >>        [<ffffffff8126540c>] store_bind+0x1ad/0x1c1
> >>        [<ffffffff8127cbb7>] dev_attr_store+0x13/0x1f
> >>        [<ffffffff8116d884>] sysfs_write_file+0xe9/0x121
> >>        [<ffffffff811145b2>] vfs_write+0x9b/0xfd
> >>        [<ffffffff811147b7>] sys_write+0x3e/0x6b
> >>        [<ffffffff813b0039>] 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 <airlied@gmail.com>
> >> Cc: Thomas Gleixner <tglx@linutronix.de>
> >> Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
> >> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> >> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >> ---
> >>  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