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

[Imre Deak]

On Fri, 2014-12-12 at 22:29 +0200, 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)
> 
> 
> 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));

Err, I just realized one mistake, the console_lock() call below needs to
be moved here, since all the assignments below need to be protected.
I'll send a v2 with this fixed.

> > > +
> > > +		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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