On Sat, Sep 22, 2012 at 10:06 PM, Greg KH <gregkh at 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 at gmail.com> >> Cc: Thomas Gleixner <tglx at linutronix.de> >> Cc: Alan Cox <alan at lxorguk.ukuu.org.uk> >> Cc: Peter Zijlstra <a.p.zijlstra at chello.nl> >> Signed-off-by: Daniel Vetter <daniel.vetter at 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 -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch
