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

Peter Hurley
2014-12-12T21:03:48Z

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.

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.

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.

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
>>>
>>
>
>