Re: [PATCH 3/3] vt: fix console lock vs. kernfs s_active lock order

Peter Hurley
2014-12-12T22:45:28Z

[ +cc Daniel because of the i915 lockdep report ]

On 12/12/2014 05:03 PM, Imre Deak wrote:
> On Fri, 2014-12-12 at 16:03 -0500, Peter Hurley wrote:
>> On 12/12/2014 03:29 PM, Imre Deak wrote:
>>> Hi Peter,
>>>
>>> thanks for your review.
>>>
>>> On Fri, 2014-12-12 at 13:32 -0500, Peter Hurley wrote:
>>>> Hi Imre,
>>>>
>>>> On 12/12/2014 11:38 AM, 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.
>>>>
>>>> This description didn't make sense to me because the driver core doesn't
>>>> try to take the console_lock. So I had to go pull the lockdep report
>>>> and I'm not sure I agree with your analysis.
>>>>
>>>> I see a three-way dependency which includes the fb atomic notifier call
>>>> chain?
>>>
>>> From the lockdep report in the bugzilla ticket I referenced, you can see
>>> the following two paths:
>>>
>>> i915_driver_load()
>>> console_lock() -> takes console_sem
>>> do_unregister_con_driver()
>>> vtconsole_deinit_device()
>>> device_remove_file()
>>> ...
>>> __kernfs_remove()
>>> kernfs_drain() ->
>>> takes s_active rwsem for the console's bind sysfs entry
>>> (tracked via kn->dep_map)
>>
>> I don't see this call chain.
>>
>> I see: (some obvious intermediate calls redacted)
>>
>> i915_driver_unload
>> do_unregister_framebuffer
>> fb_notifier_call_chain
>> fbcon_event_notify
>> do_unregister_con_driver
>> device_remove_file
>> sysfs_addrm_finish
>>
>> which has console_lock => fb notifier lock => kernfs lock dependency.
>
> Ah, right, there are two dmesg logs in the bug report, your sequence is
> the first one [1] and I talked about the second [2].
>
> [1] https://bugs.freedesktop.org/attachment.cgi?id=87716
> [2] https://bugs.freedesktop.org/attachment.cgi?id=107602

Which is why your evidence/justification should be in the commit message
rather than presuming that it's self-evident from a reference to a bug report.

However, it doesn't seem to me that reference [2] provides more evidence for
changing VT; why is i915 the only drm driver trying to unload the vga console
from it's .load method? Without that, the problem is back to fbcon.

[Aside: istm, the design error here was to expose bind/unbind as sysfs hooks
from the VT console code, but that doesn't look remediable. ]

Regards,
Peter Hurley

>> My point being that this occurs only because the fb notifier call chain
>> requires console_lock => fb notifier lock dependency ordering; that and
>> fbcon assumes that it can do whatever in the notifier.
>
> Yes this is correct and I don't see any place where we would have the
> opposite order.
>
> The lockdep report in [1] is really just the same problem I described
> which is the s_active->console_lock dependency:
>
> CPU0 CPU1
> ---- ----
> lock((fb_notifier_list).rwsem);
> lock(console_lock);
> lock((fb_notifier_list).rwsem);
> lock(s_active#114);
> *** DEADLOCK ***
>
> This can lead to a deadlock only because s_active already depends on
> console_lock. After removing this dependency I can't see any other way
> these locks could deadlock.
>
>> I would like to see more thorough justification for why this change
>> belongs in vt, and not in fbcon. That was my point about the 3-way
>> dependency.
>
> Since it's really about the dependency between console_lock and s_active
> and fbcon is not involved in all code paths where we take these locks
> (like in the [2] case) but vt is.
>
> --Imre
>
>>
>> Regards,
>> Peter Hurley
>>
>>
>>> vfs_write() for the above console bind sysfs entry
>>> kernfs_fop_write()
>>> kernfs_get_active() ->
>>> takes s_active rwsem for the above sysfs entry
>>> ...
>>> store_bind() -> takes console_sem
>>>
>>> So you have console_sem->s_active ordering on one path and
>>> s_active->console_sem ordering on the other.
>>>
>>> This patch gets rid of the ordering problem and the related lockdep
>>> warning.
>>>
>>> --Imre
>>>
>>>>
>>>> Regards,
>>>> Peter Hurley
>>>>
>>>>> 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.
>>>>>
>>>>> Reference: https://bugs.freedesktop.org/show_bug.cgi?id=70523
>>>>> Signed-off-by: Imre Deak <imre.deak@intel.com>
>>>>> ---
>>>>> drivers/tty/vt/vt.c | 51 +++++++++++++++++++++++++++++++++++++++++----------
>>>>> 1 file changed, 41 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
>>>>> index 5dd1880..b9edc77 100644
>>>>> --- a/drivers/tty/vt/vt.c
>>>>> +++ b/drivers/tty/vt/vt.c
>>>>> @@ -108,6 +108,7 @@
>>>>> #define CON_DRIVER_FLAG_MODULE 1
>>>>> #define CON_DRIVER_FLAG_INIT 2
>>>>> #define CON_DRIVER_FLAG_ATTR 4
>>>>> +#define CON_DRIVER_FLAG_ZOMBIE 8
>>>>>
>>>>> struct con_driver {
>>>>> const struct consw *con;
>>>>> @@ -153,6 +154,7 @@ static int set_vesa_blanking(char __user *p);
>>>>> static void set_cursor(struct vc_data *vc);
>>>>> static void hide_cursor(struct vc_data *vc);
>>>>> static void console_callback(struct work_struct *ignored);
>>>>> +static void con_driver_unregister_callback(struct work_struct *ignored);
>>>>> static void blank_screen_t(unsigned long dummy);
>>>>> static void set_palette(struct vc_data *vc);
>>>>>
>>>>> @@ -180,6 +182,7 @@ static int blankinterval = 10*60;
>>>>> core_param(consoleblank, blankinterval, int, 0444);
>>>>>
>>>>> static DECLARE_WORK(console_work, console_callback);
>>>>> +static DECLARE_WORK(con_driver_unregister_work, con_driver_unregister_callback);
>>>>>
>>>>> /*
>>>>> * fg_console is the current virtual console,
>>>>> @@ -3597,7 +3600,8 @@ static int do_register_con_driver(const struct consw *csw, int first, int last)
>>>>> for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
>>>>> con_driver = &registered_con_driver[i];
>>>>>
>>>>> - if (con_driver->con == NULL) {
>>>>> + if (con_driver->con == NULL &&
>>>>> + !(con_driver->flag & CON_DRIVER_FLAG_ZOMBIE)) {
>>>>> con_driver->con = csw;
>>>>> con_driver->desc = desc;
>>>>> con_driver->node = i;
>>>>> @@ -3660,16 +3664,10 @@ int do_unregister_con_driver(const struct consw *csw)
>>>>>
>>>>> if (con_driver->con == csw &&
>>>>> con_driver->flag & CON_DRIVER_FLAG_MODULE) {
>>>>> - vtconsole_deinit_device(con_driver);
>>>>> - device_destroy(vtconsole_class,
>>>>> - MKDEV(0, con_driver->node));
>>>>> con_driver->con = NULL;
>>>>> - con_driver->desc = NULL;
>>>>> - con_driver->dev = NULL;
>>>>> - con_driver->node = 0;
>>>>> - con_driver->flag = 0;
>>>>> - con_driver->first = 0;
>>>>> - con_driver->last = 0;
>>>>> + con_driver->flag = CON_DRIVER_FLAG_ZOMBIE;
>>>>> + schedule_work(&con_driver_unregister_work);
>>>>> +
>>>>> return 0;
>>>>> }
>>>>> }
>>>>> @@ -3678,6 +3676,39 @@ int do_unregister_con_driver(const struct consw *csw)
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(do_unregister_con_driver);
>>>>>
>>>>> +static void con_driver_unregister_callback(struct work_struct *ignored)
>>>>> +{
>>>>> + int i;
>>>>> +
>>>>> + console_lock();
>>>>> +
>>>>> + for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
>>>>> + struct con_driver *con_driver = &registered_con_driver[i];
>>>>> +
>>>>> + if (!(con_driver->flag & CON_DRIVER_FLAG_ZOMBIE))
>>>>> + continue;
>>>>> +
>>>>> + console_unlock();
>>>>> +
>>>>> + vtconsole_deinit_device(con_driver);
>>>>> + device_destroy(vtconsole_class, MKDEV(0, con_driver->node));
>>>>> +
>>>>> + if (WARN_ON_ONCE(con_driver->con))
>>>>> + con_driver->con = NULL;
>>>>> + con_driver->desc = NULL;
>>>>> + con_driver->dev = NULL;
>>>>> + con_driver->node = 0;
>>>>> + WARN_ON_ONCE(con_driver->flag != CON_DRIVER_FLAG_ZOMBIE);
>>>>> + con_driver->flag = 0;
>>>>> + con_driver->first = 0;
>>>>> + con_driver->last = 0;
>>>>> +
>>>>> + console_lock();
>>>>> + }
>>>>> +
>>>>> + console_unlock();
>>>>> +}
>>>>> +
>>>>> /*
>>>>> * If we support more console drivers, this function is used
>>>>> * when a driver wants to take over some existing consoles
>>>>>
>>>>
>>>
>>>
>>
>
>