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

From: Imre Deak
Date: Fri Dec 12 2014 - 15:30:00 EST

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)


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@xxxxxxxxx>
> > ---
> > 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
> >
>


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