Message-ID: <5490491E.70102@hurleysoftware.com>
Date:	Tue, 16 Dec 2014 10:00:46 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	imre.deak@...el.com
CC:	Daniel Vetter <daniel@...ll.ch>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] vt: fix console lock vs. kernfs s_active lock
 order

On 12/16/2014 09:38 AM, Imre Deak wrote:
> On Tue, 2014-12-16 at 07:50 -0500, Peter Hurley wrote:
>> On 12/16/2014 05:23 AM, Imre Deak wrote:
>>> On Tue, 2014-12-16 at 08:53 +0100, Daniel Vetter wrote:
>>>> On Tue, Dec 16, 2014 at 12:16:01AM +0200, Imre Deak wrote:
>>>>> Currently there is a lock order problem between the console lock and the
>>>>> kernfs s_active lock of the console driver's bind sysfs entry. When
>>>>> writing to the sysfs entry the lock order is first s_active then console
>>>>> lock, when unregistering the console driver via
>>>>> do_unregister_con_driver() the order is the opposite. See the below
>>>>> bugzilla reference for one instance of a lockdep backtrace.
>>>>>
>>>>> Fix this by unregistering the console driver from a deferred work, where
>>>>> we can safely drop the console lock while unregistering the device and
>>>>> corresponding sysfs entries (which in turn acquire s_active). Note that
>>>>> we have to keep the console driver slot in the registered_con_driver
>>>>> array reserved for the driver that's being unregistered until it's fully
>>>>> removed. Otherwise a concurrent call to do_register_con_driver could
>>>>> try to reuse the same slot and fail when registering the corresponding
>>>>> device with a minor index that's still in use.
>>>>>
>>>>> Note that the referenced bug report contains two dmesg logs with two
>>>>> distinct lockdep reports: [1] is about a locking scenario involving
>>>>> s_active, console_lock and the fb_notifier list lock, while [2] is
>>>>> about a locking scenario involving only s_active and console_lock.
>>>>> In [1] locking fb_notifier triggers the lockdep warning only because
>>>>> of its dependence on console_lock, otherwise case [1] is the same
>>>>> s_active<->console_lock dependency problem fixed by this patch.
>>>>> Before this change we have the following locking scenarios involving
>>>>> the 3 locks:
>>>>>
>>>>> a) via do_unregister_framebuffer()->...->do_unregister_con_driver():
>>>>>    1. console lock 2. fb_notifier lock 3. s_active lock
>>>>> b) for example via give_up_console()->do_unregister_con_driver():
>>>>>    1. console lock 2. s_active lock
>>>>> c) via vt_bind()/vt_unbind():
>>>>>    1. s_active lock 2. console lock
>>>>>
>>>>> Since c) is the console bind sysfs entry's write code path we can't
>>>>> change the locking order there. We can only fix this issue by removing
>>>>> s_active's dependence on the other two locks in a) and b). We can do
>>>>> this only in the vt code which owns the corresponding sysfs entry, so
>>>>> that after the change we have:
>>>>>
>>>>> a) 1. console lock 2. fb_notifier lock
>>>>> b) 1. console lock
>>>>> c) 1. s_active lock 2. console lock
>>>>> d) in the new con_driver_unregister_callback():
>>>>>    1. s_active lock
>>>>>
>>>>> [1] https://bugs.freedesktop.org/attachment.cgi?id=87716
>>>>> [2] https://bugs.freedesktop.org/attachment.cgi?id=107602
>>>>>
>>>>> v2:
>>>>> - get console_lock earlier in con_driver_unregister_callback(), so we
>>>>>   protect the following console driver field assignments there
>>>>> - add code coment explaining the reason for deferring the sysfs entry
>>>>>   removal
>>>>> - add a third paragraph to the commit message explaining why there are
>>>>>   two distinct lockdep reports and that this issue is independent of
>>>>>   fb/fbcon. (Peter Hurley)
>>>>> v3:
>>>>> - clarify further the third paragraph
>>>>> v4:
>>>>> - rebased on v4 of patch 1/3
>>>>>
>>>>> Reference: https://bugs.freedesktop.org/show_bug.cgi?id=70523
>>>>> Signed-off-by: Imre Deak <imre.deak@...el.com>
>>>>
>>>> So the normal/simple solution would be to remove the con driver from the
>>>> registration while holding required locks, but destroy sysfs stuff after
>>>> the critical section.
>>>>
>>>> The problem is that console unbind/bind/reg/unreg are all done with the
>>>> locks held already, and the reason for that is the fbcon/fbdev notifier
>>>> chain. You can read up on the story by chasing the commits that added the
>>>> various locked do_* functions over the past years.
>>>>
>>>> Short story is that the notifier chain has it's own mutex and any call
>>>> mediated through it introces that lock into the chain, which creates a
>>>> massive pile of additional depencies. The only solution is to grab _all_
>>>> required locks outside of that notifier chain, which means we've spent a
>>>> lot of patches growing the scope of console_lock. Which is bad, since
>>>> anything holding console_lock can't get dmesg lines out when it dies.
>>>
>>> These are independent issues from what this patch fixes. It doesn't try
>>> to grow the scope of console_lock either, it makes the sysfs s_active
>>> lock independent of console_lock.
>>>
>>>> This patch here is another step into the wrong direction imo. It's also
>>>> for a feature that mere users should never use (even though it's in
>>>> sysfs). Heck it's a regression which has been introduced by pulling
>>>> console_lock out&up, before that effort sysfs files worked correctly and
>>>> implemented locking as described.
>>>
>>> It doesn't matter where you take console_lock, since it's fixed where
>>> s_active is taken (in the vfs code before the sysfs entry's write hook
>>> is called). So again this issue is not related to the growing scope of
>>> console_lock. This fix makes the s_active lock independent of
>>> console_lock, so I don't see why it's a step in the wrong direction.
>>>
>>>> The right solution imo here is to at least cut up the fbdev notifier into
>>>> the different uses so that the locking artificial locking inversions go
>>>> away. Most of the calls are used to go from fbdev core (fbmem.c) to the
>>>> fbcon.c. Apparently someone though it would be great to allow fb.ko and
>>>> fbcon.ko to be able to load in any order whatsoever. Then there's various
>>>> other calls that go the other direction (e.g. fbcon calling into backlight
>>>> logic) and those introduce the locking inversion. So if we split things
>>>> into 2 notifier chais, one to exclusively call into fbcon and the other
>>>> for everything else we could revert all the patche that move console_lock
>>>> out and fix this bug here properly.
>>>>
>>>> So NACK from me for this.
>>>
>>> I think I proved it in the commit message that this issue is independent
>>> of fbcon/fbdev, so refactoring these will not solve it. This patch fixes
>>> an issue in vt and while your points may be valid, they are not really
>>> about this issue or how the patch fixes it.
>>
>> I think you may have missed Daniel's point. Which is what I was trying to
>> point out earlier but in an overly terse manner.
> 
> I did get what Daniel said and they are valid points. They require more
> work though and since things are broken in the vt code right now we
> should try to fix it there, instead of waiting for those planned changes
> elsewhere to take place.
> 
> The fix will be anyway the same in principal even after Daniel's planned
> rework for fbcon/fbdev: not holding the console_lock while freeing the
> sysfs entries.

Oh, I didn't know Daniel was planning to rework fbcon/fbdev.

And these are not 'the same in principal'. Deferring to a worker thread
is not a magic bullet; it allows for race conditions that don't otherwise
exist.

> The only difference is that this happens in a deferred
> work in my patch vs. the same thread after the planned refactoring.
> 
>> If you start by just moving the sysfs teardown outside the console_lock
>> (but still in the same thread of execution), then the direct lock inversion
>> between console_lock and the sysfs lock goes away.
>>
>> However, you'll now realize that you can't move the sysfs teardown outside
>> the console lock because fbcon is doing teardown from inside its notifier,
>> which means that there would be a lock inversion between the console lock
>> and the notifier lock.
>>
>> Which is why we're pointing out that the real problem here is the
>> fb notifier call chain lock.
> 
> Right, I agree that the ideal solution would be to not have a deferred
> work for this, but I don't know how much refactoring that requires
> elsewhere. The fix is technically correct as Daniel pointed out and imo
> it's simple enough to apply it while the bigger refactoring takes place.
> That way we can have a working way to reload modules, which is broken
> atm.

Fine. Just another expedient fix piled on top of other expedient fixes
that go back past 3.9 with no end in sight.

Regards,
Peter Hurley
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