[PATCH 1/2] [RESEND] console: use might_sleep in console_lock

[PATCH 1/2] [RESEND] console: use might_sleep in console_lock

[Daniel Vetter]

Instead of BUG_ON(in_interrupt()), since that doesn't check for all
the newfangled stuff like preempt.

Note that this is valid since the console_sem is essentially used like
a real mutex with only two twists:
- we allow trylock from hardirq context
- across suspend/resume we lock the logical console_lock, but drop the
  semaphore protecting the locking state.

Now that doesn't guarantee that no one is playing tricks in
single-thread atomic contexts at suspend/resume/boot time, but
- I couldn't find anything suspicious with some grepping,
- might_sleep shouldn't die,
- and I think the upside of catching more potential issues is worth
  the risk of getting a might_sleep backtrace that would have been
  save (and then dealing with that fallout).

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 |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 66a2ea3..ed9af6a 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1909,7 +1909,8 @@ static int __cpuinit console_cpu_notify(struct notifier_block *self,
  */
 void console_lock(void)
 {
-	BUG_ON(in_interrupt());
+	might_sleep();
+
 	down(&console_sem);
 	if (console_suspended)
 		return;
-- 
1.7.10.4

[PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Daniel Vetter]

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 ***

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(+)

diff --git a/kernel/printk.c b/kernel/printk.c
index ed9af6a..ab2ab24 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -87,6 +87,12 @@ static DEFINE_SEMAPHORE(console_sem);
 struct console *console_drivers;
 EXPORT_SYMBOL_GPL(console_drivers);
 
+#ifdef CONFIG_LOCKDEP
+struct lockdep_map console_lock_dep_map = {
+	.name = "console_lock"
+};
+#endif
+
 /*
  * This is used for debugging the mess that is the VT code by
  * keeping track if we have the console semaphore held. It's
@@ -1916,6 +1922,7 @@ void console_lock(void)
 		return;
 	console_locked = 1;
 	console_may_schedule = 1;
+	mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);
 }
 EXPORT_SYMBOL(console_lock);
 
@@ -1937,6 +1944,7 @@ int console_trylock(void)
 	}
 	console_locked = 1;
 	console_may_schedule = 0;
+	mutex_acquire(&console_lock_dep_map, 0, 1, _RET_IP_);
 	return 1;
 }
 EXPORT_SYMBOL(console_trylock);
@@ -2097,6 +2105,7 @@ skip:
 		local_irq_restore(flags);
 	}
 	console_locked = 0;
+	mutex_release(&console_lock_dep_map, 1, _RET_IP_);
 
 	/* Release the exclusive_console once it is used */
 	if (unlikely(exclusive_console))
-- 
1.7.10.4

Re: [PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Jani Nikula]

On Tue, 18 Sep 2012, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> +#ifdef CONFIG_LOCKDEP
> +struct lockdep_map console_lock_dep_map = {
> +	.name = "console_lock"
> +};
> +#endif

static?


BR,
Jani.

Re: [PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Daniel Vetter]

On Tue, Sep 18, 2012 at 10:33:28AM +0300, Jani Nikula wrote:
> On Tue, 18 Sep 2012, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > +#ifdef CONFIG_LOCKDEP
> > +struct lockdep_map console_lock_dep_map = {
> > +	.name = "console_lock"
> > +};
> > +#endif
> 
> static?

Yeah, static. I'm travelling atm, so will take a while to resend ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[PATCH] console: implement lockdep support for console_lock

[Daniel Vetter]

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(+)

diff --git a/kernel/printk.c b/kernel/printk.c
index ed9af6a..e5c6dba 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -87,6 +87,12 @@ static DEFINE_SEMAPHORE(console_sem);
 struct console *console_drivers;
 EXPORT_SYMBOL_GPL(console_drivers);
 
+#ifdef CONFIG_LOCKDEP
+static struct lockdep_map console_lock_dep_map = {
+	.name = "console_lock"
+};
+#endif
+
 /*
  * This is used for debugging the mess that is the VT code by
  * keeping track if we have the console semaphore held. It's
@@ -1916,6 +1922,7 @@ void console_lock(void)
 		return;
 	console_locked = 1;
 	console_may_schedule = 1;
+	mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);
 }
 EXPORT_SYMBOL(console_lock);
 
@@ -1937,6 +1944,7 @@ int console_trylock(void)
 	}
 	console_locked = 1;
 	console_may_schedule = 0;
+	mutex_acquire(&console_lock_dep_map, 0, 1, _RET_IP_);
 	return 1;
 }
 EXPORT_SYMBOL(console_trylock);
@@ -2097,6 +2105,7 @@ skip:
 		local_irq_restore(flags);
 	}
 	console_locked = 0;
+	mutex_release(&console_lock_dep_map, 1, _RET_IP_);
 
 	/* Release the exclusive_console once it is used */
 	if (unlikely(exclusive_console))
-- 
1.7.10.4

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

[Greg KH]

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?

thanks,

greg k-h

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

[Daniel Vetter]

On Sat, Sep 22, 2012 at 01:06:29PM -0700, 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: [<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 didn't know who would be the relevant maintainer, so just spammed a few
people. Would be awesome if you could merge these patches for 3.7, and at
least Alan Cox seems to like them:

http://marc.info/?l=linux-fbdev&m=134564125601147&w=1

Thanks, Daniel
> 
> thanks,
> 
> greg k-h

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Peter Zijlstra]

On Tue, 2012-09-18 at 01:03 +0200, Daniel Vetter wrote:
> - 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. 

Not particularly evil, just plain broken.

See this series:

  http://marc.info/?l=linux-kernel&m=132446644123326&w=2

In particular see patches 1-4. The problem with lockdep is that lockdep
uses printk and printk's recursion stuff is utterly broken.

Console drivers being on crack simply doesn't help :-) Note that I would
never recommend drm/ksm to anybody who really cares about their console
output.

Re: [PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Peter Zijlstra]

On Mon, 2012-09-24 at 14:17 +0200, Peter Zijlstra wrote:
> On Tue, 2012-09-18 at 01:03 +0200, Daniel Vetter wrote:
> > - 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. 
> 
> Not particularly evil, just plain broken.
> 
> See this series:
> 
>   http://marc.info/?l=linux-kernel&m=132446644123326&w=2
> 
> In particular see patches 1-4. The problem with lockdep is that lockdep
> uses printk and printk's recursion stuff is utterly broken.
> 
> Console drivers being on crack simply doesn't help :-) Note that I would
> never recommend drm/ksm to anybody who really cares about their console
> output.

Also, don't cross-post with a subscribe only list.

Re: [PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Daniel Vetter]

On Mon, Sep 24, 2012 at 2:24 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, 2012-09-24 at 14:17 +0200, Peter Zijlstra wrote:
>> On Tue, 2012-09-18 at 01:03 +0200, Daniel Vetter wrote:
>> > - 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.
>>
>> Not particularly evil, just plain broken.
>>
>> See this series:
>>
>>   http://marc.info/?l=linux-kernel&m=132446644123326&w=2
>>
>> In particular see patches 1-4. The problem with lockdep is that lockdep
>> uses printk and printk's recursion stuff is utterly broken.
>>
>> Console drivers being on crack simply doesn't help :-) Note that I would
>> never recommend drm/ksm to anybody who really cares about their console
>> output.

I've read through the patches and I'm hoping you don't volunteer me to
pick these up ... ;-) But there doesn't seem to be anything that would
get worse through this lockdep annotation patch, right?

> Also, don't cross-post with a subscribe only list.

Sorry, forgot about this, I'll try to fix this - we really need an
intel-gfx that's open to people not subscribed to it.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [PATCH 2/2] [RESEND] console: implement lockdep support for console_lock

[Peter Zijlstra]

On Mon, 2012-09-24 at 14:54 +0200, Daniel Vetter wrote:
> I've read through the patches and I'm hoping you don't volunteer me to
> pick these up ... ;-) 

Worth a try, right? :-)

> But there doesn't seem to be anything that would
> get worse through this lockdep annotation patch, right? 

No indeed, your patch looks fine, I just wanted to comment on the
printk() thing.

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

[Daniel Vetter]

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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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

[Greg KH]

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

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

[Daniel Vetter]

On Tue, Oct 2, 2012 at 3:28 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> 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.

Hey, was worth a shot ;-) Yeah, if you can pick it up for 3.8, that
would be nice, since the patches have been floating for a while by now
...

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch