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

Imre Deak
2014-12-12T22:03:55Z

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

>
> 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
> >>>
> >>
> >
> >
>