Date:	Tue, 16 Dec 2014 18:22:19 +0200
From:	Imre Deak <imre.deak@...el.com>
To:	Peter Hurley <peter@...leysoftware.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 Tue, 2014-12-16 at 10:00 -0500, Peter Hurley wrote:
> 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.

True, there is a race with the current approach, but it's there even if
you didn't have a deferred work. As soon as you drop the console_lock
you'd have to make sure the given slot in registered_con_driver isn't
reused until you free the underlying struct device. One way to do that
is using the new flag I added.

> > 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.

I'm also happy to look into narrowing down the scope of console_lock in
fbdev/fbcon as was suggested. But doing that as a follow-up to this
change still makes sense to me since it will take more time and have the
risk of regressions that are not related to what this change fixes.

--Imre

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