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 ...
Thanks, Daniel