Re: [PATCH v3 3/8] drm/fb_helper: Introduce hotplug_suspend/resume()

From: Daniel Vetter
Date: Mon Aug 06 2018 - 04:43:30 EST

On Mon, Jul 30, 2018 at 08:39:48PM -0400, Lyude Paul wrote:
> I'm sure I don't need to tell you that fb_helper's locking is a mess.
> That being said; fb_helper's locking mess can seriously complicate the
> runtime suspend/resume operations of drivers because it can invoke
> atomic commits and connector probing from anywhere that calls
> drm_fb_helper_hotplug_event(). Since most drivers use
> drm_fb_helper_output_poll_changed() as their output_poll_changed
> handler, this can happen in every single context that can fire off a
> hotplug event. An example:

Uh, I think this ever more looks like a serious rabbit hole between
nouveau and rpm. It looks like nouveau tries to use rpm as the outermost
lock, surrounding every other drm lock there is. That doesn't really work,
and means you'll keep fighting drm core locking forever.

All other rpm supporting drivers (i915, and the pile of armsoc ones) push
their rpm_get/put dance way down into the low-level callbacks. That then
allows you to fix all the trouble with probing, i2x, dp_aux and stuff
abitrarily nesting (since rpm itself is refcounted).

But I'm not really sure why nouveau is different here.
-Daniel

>
> [ 246.669625] INFO: task kworker/4:0:37 blocked for more than 120 seconds.
> [ 246.673398] Not tainted 4.18.0-rc5Lyude-Test+ #2
> [ 246.675271] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 246.676527] kworker/4:0 D 0 37 2 0x80000000
> [ 246.677580] Workqueue: events output_poll_execute [drm_kms_helper]
> [ 246.678704] Call Trace:
> [ 246.679753] __schedule+0x322/0xaf0
> [ 246.680916] schedule+0x33/0x90
> [ 246.681924] schedule_preempt_disabled+0x15/0x20
> [ 246.683023] __mutex_lock+0x569/0x9a0
> [ 246.684035] ? kobject_uevent_env+0x117/0x7b0
> [ 246.685132] ? drm_fb_helper_hotplug_event.part.28+0x20/0xb0 [drm_kms_helper]
> [ 246.686179] mutex_lock_nested+0x1b/0x20
> [ 246.687278] ? mutex_lock_nested+0x1b/0x20
> [ 246.688307] drm_fb_helper_hotplug_event.part.28+0x20/0xb0 [drm_kms_helper]
> [ 246.689420] drm_fb_helper_output_poll_changed+0x23/0x30 [drm_kms_helper]
> [ 246.690462] drm_kms_helper_hotplug_event+0x2a/0x30 [drm_kms_helper]
> [ 246.691570] output_poll_execute+0x198/0x1c0 [drm_kms_helper]
> [ 246.692611] process_one_work+0x231/0x620
> [ 246.693725] worker_thread+0x214/0x3a0
> [ 246.694756] kthread+0x12b/0x150
> [ 246.695856] ? wq_pool_ids_show+0x140/0x140
> [ 246.696888] ? kthread_create_worker_on_cpu+0x70/0x70
> [ 246.697998] ret_from_fork+0x3a/0x50
> [ 246.699034] INFO: task kworker/0:1:60 blocked for more than 120 seconds.
> [ 246.700153] Not tainted 4.18.0-rc5Lyude-Test+ #2
> [ 246.701182] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 246.702278] kworker/0:1 D 0 60 2 0x80000000
> [ 246.703293] Workqueue: pm pm_runtime_work
> [ 246.704393] Call Trace:
> [ 246.705403] __schedule+0x322/0xaf0
> [ 246.706439] ? wait_for_completion+0x104/0x190
> [ 246.707393] schedule+0x33/0x90
> [ 246.708375] schedule_timeout+0x3a5/0x590
> [ 246.709289] ? mark_held_locks+0x58/0x80
> [ 246.710208] ? _raw_spin_unlock_irq+0x2c/0x40
> [ 246.711222] ? wait_for_completion+0x104/0x190
> [ 246.712134] ? trace_hardirqs_on_caller+0xf4/0x190
> [ 246.713094] ? wait_for_completion+0x104/0x190
> [ 246.713964] wait_for_completion+0x12c/0x190
> [ 246.714895] ? wake_up_q+0x80/0x80
> [ 246.715727] ? get_work_pool+0x90/0x90
> [ 246.716649] flush_work+0x1c9/0x280
> [ 246.717483] ? flush_workqueue_prep_pwqs+0x1b0/0x1b0
> [ 246.718442] __cancel_work_timer+0x146/0x1d0
> [ 246.719247] cancel_delayed_work_sync+0x13/0x20
> [ 246.720043] drm_kms_helper_poll_disable+0x1f/0x30 [drm_kms_helper]
> [ 246.721123] nouveau_pmops_runtime_suspend+0x3d/0xb0 [nouveau]
> [ 246.721897] pci_pm_runtime_suspend+0x6b/0x190
> [ 246.722825] ? pci_has_legacy_pm_support+0x70/0x70
> [ 246.723737] __rpm_callback+0x7a/0x1d0
> [ 246.724721] ? pci_has_legacy_pm_support+0x70/0x70
> [ 246.725607] rpm_callback+0x24/0x80
> [ 246.726553] ? pci_has_legacy_pm_support+0x70/0x70
> [ 246.727376] rpm_suspend+0x142/0x6b0
> [ 246.728185] pm_runtime_work+0x97/0xc0
> [ 246.728938] process_one_work+0x231/0x620
> [ 246.729796] worker_thread+0x44/0x3a0
> [ 246.730614] kthread+0x12b/0x150
> [ 246.731395] ? wq_pool_ids_show+0x140/0x140
> [ 246.732202] ? kthread_create_worker_on_cpu+0x70/0x70
> [ 246.732878] ret_from_fork+0x3a/0x50
> [ 246.733768] INFO: task kworker/4:2:422 blocked for more than 120 seconds.
> [ 246.734587] Not tainted 4.18.0-rc5Lyude-Test+ #2
> [ 246.735393] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 246.736113] kworker/4:2 D 0 422 2 0x80000080
> [ 246.736789] Workqueue: events_long drm_dp_mst_link_probe_work [drm_kms_helper]
> [ 246.737665] Call Trace:
> [ 246.738490] __schedule+0x322/0xaf0
> [ 246.739250] schedule+0x33/0x90
> [ 246.739908] rpm_resume+0x19c/0x850
> [ 246.740750] ? finish_wait+0x90/0x90
> [ 246.741541] __pm_runtime_resume+0x4e/0x90
> [ 246.742370] nv50_disp_atomic_commit+0x31/0x210 [nouveau]
> [ 246.743124] drm_atomic_commit+0x4a/0x50 [drm]
> [ 246.743775] restore_fbdev_mode_atomic+0x1c8/0x240 [drm_kms_helper]
> [ 246.744603] restore_fbdev_mode+0x31/0x140 [drm_kms_helper]
> [ 246.745373] drm_fb_helper_restore_fbdev_mode_unlocked+0x54/0xb0 [drm_kms_helper]
> [ 246.746220] drm_fb_helper_set_par+0x2d/0x50 [drm_kms_helper]
> [ 246.746884] drm_fb_helper_hotplug_event.part.28+0x96/0xb0 [drm_kms_helper]
> [ 246.747675] drm_fb_helper_output_poll_changed+0x23/0x30 [drm_kms_helper]
> [ 246.748544] drm_kms_helper_hotplug_event+0x2a/0x30 [drm_kms_helper]
> [ 246.749439] nv50_mstm_hotplug+0x15/0x20 [nouveau]
> [ 246.750111] drm_dp_send_link_address+0x177/0x1c0 [drm_kms_helper]
> [ 246.750764] drm_dp_check_and_send_link_address+0xa8/0xd0 [drm_kms_helper]
> [ 246.751602] drm_dp_mst_link_probe_work+0x51/0x90 [drm_kms_helper]
> [ 246.752314] process_one_work+0x231/0x620
> [ 246.752979] worker_thread+0x44/0x3a0
> [ 246.753838] kthread+0x12b/0x150
> [ 246.754619] ? wq_pool_ids_show+0x140/0x140
> [ 246.755386] ? kthread_create_worker_on_cpu+0x70/0x70
> [ 246.756162] ret_from_fork+0x3a/0x50
> [ 246.756847]
> Showing all locks held in the system:
> [ 246.758261] 3 locks held by kworker/4:0/37:
> [ 246.759016] #0: 00000000f8df4d2d ((wq_completion)"events"){+.+.}, at: process_one_work+0x1b3/0x620
> [ 246.759856] #1: 00000000e6065461 ((work_completion)(&(&dev->mode_config.output_poll_work)->work)){+.+.}, at: process_one_work+0x1b3/0x620
> [ 246.760670] #2: 00000000cb66735f (&helper->lock){+.+.}, at: drm_fb_helper_hotplug_event.part.28+0x20/0xb0 [drm_kms_helper]
> [ 246.761516] 2 locks held by kworker/0:1/60:
> [ 246.762274] #0: 00000000fff6be0f ((wq_completion)"pm"){+.+.}, at: process_one_work+0x1b3/0x620
> [ 246.762982] #1: 000000005ab44fb4 ((work_completion)(&dev->power.work)){+.+.}, at: process_one_work+0x1b3/0x620
> [ 246.763890] 1 lock held by khungtaskd/64:
> [ 246.764664] #0: 000000008cb8b5c3 (rcu_read_lock){....}, at: debug_show_all_locks+0x23/0x185
> [ 246.765588] 5 locks held by kworker/4:2/422:
> [ 246.766440] #0: 00000000232f0959 ((wq_completion)"events_long"){+.+.}, at: process_one_work+0x1b3/0x620
> [ 246.767390] #1: 00000000bb59b134 ((work_completion)(&mgr->work)){+.+.}, at: process_one_work+0x1b3/0x620
> [ 246.768154] #2: 00000000cb66735f (&helper->lock){+.+.}, at: drm_fb_helper_restore_fbdev_mode_unlocked+0x4c/0xb0 [drm_kms_helper]
> [ 246.768966] #3: 000000004c8f0b6b (crtc_ww_class_acquire){+.+.}, at: restore_fbdev_mode_atomic+0x4b/0x240 [drm_kms_helper]
> [ 246.769921] #4: 000000004c34a296 (crtc_ww_class_mutex){+.+.}, at: drm_modeset_backoff+0x8a/0x1b0 [drm]
> [ 246.770839] 1 lock held by dmesg/1038:
> [ 246.771739] 2 locks held by zsh/1172:
> [ 246.772650] #0: 00000000836d0438 (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x37/0x40
> [ 246.773680] #1: 000000001f4f4d48 (&ldata->atomic_read_lock){+.+.}, at: n_tty_read+0xc1/0x870
>
> [ 246.775522] =============================================
>
> Because of this, there's an unreasonable number of places that drm
> drivers would need to insert special handling to prevent trying to
> resume the device from all of these contexts that can deadlock. It's
> difficult even to try synchronizing with fb_helper in these contexts as
> well, since any of them could introduce a deadlock by waiting to acquire
> the top-level fb_helper mutex, while it's being held by another thread
> that might potentially call down to pm_runtime_get_sync().
>
> Luckily-there's no actual reason we need to allow fb_helper to handle
> hotplugging at all when runtime suspending a device. If a hotplug
> happens during a runtime suspend operation, there's no reason the driver
> can't just re-enable fbcon's hotplug handling and bring it up to speed
> with hotplugging events it may have missed by calling
> drm_fb_helper_hotplug_event().
>
> So, let's make this easy and just add helpers to handle disabling and
> enabling fb_helper connector probing() without having to potentially
> wait on fb_helper to finish it's work. This will let us fix the runtime
> suspend/resume deadlocks that we've been experiencing with nouveau,
> along with being able to fix some of the incorrect runtime PM core
> interaction that other DRM drivers currently perform to work around
> these issues.
>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Lukas Wunner <lukas@xxxxxxxxx>
> Cc: Karol Herbst <karolherbst@xxxxxxxxx>
> ---
> drivers/gpu/drm/drm_fb_helper.c | 65 +++++++++++++++++++++++++++++++++
> include/drm/drm_fb_helper.h | 20 ++++++++++
> 2 files changed, 85 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 2ee1eaa66188..28d59befbc92 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -2733,6 +2733,66 @@ int drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel)
> }
> EXPORT_SYMBOL(drm_fb_helper_initial_config);
>
> +/**
> + * drm_fb_helper_resume_hotplug - Allow fb_helper to handle probing new
> + * connectors again, and bring it up to date
> + * with the current device's connector status
> + * fb_helper: driver-allocated fbdev helper, can be NULL
> + *
> + * Allow fb_helper to react to connector status changes again after having
> + * been disabled through drm_fb_helper_suspend_hotplug(). This also schedules
> + * a fb_helper hotplug event to bring fb_helper up to date with the current
> + * status of the DRM device's connectors.
> + */
> +void
> +drm_fb_helper_resume_hotplug(struct drm_fb_helper *fb_helper)
> +{
> + fb_helper->hotplug_suspended = false;
> + drm_fb_helper_hotplug_event(fb_helper);
> +}
> +EXPORT_SYMBOL(drm_fb_helper_resume_hotplug);
> +
> +/**
> + * drm_fb_helper_suspend_hotplug - Attempt to temporarily inhibit fb_helper's
> + * ability to respond to connector changes
> + * without waiting
> + * @fb_helper: driver-allocated fbdev helper, can be NULL
> + *
> + * Temporarily prevent fb_helper from being able to handle connector changes,
> + * but only if it isn't busy doing something else. The connector probing
> + * routines in fb_helper can potentially grab any modesetting lock imaginable,
> + * which dramatically complicates the runtime suspend process. This helper can
> + * be used to simplify the process of runtime suspend by allowing the driver
> + * to disable unpredictable fb_helper operations as early as possible, without
> + * requiring that we try waiting on fb_helper (as this could lead to very
> + * difficult to solve deadlocks with runtime suspend code if fb_helper ends up
> + * needing to acquire a runtime PM reference).
> + *
> + * This call should be put at the very start of a driver's runtime suspend
> + * operation if desired. The driver must be responsible for re-enabling
> + * fb_helper hotplug handling when normal hotplug detection becomes available
> + * on the device again. This will usually happen if runtime suspend is
> + * aborted, or when the device is runtime resumed.
> + *
> + * Returns: true if hotplug handling was disabled, false if disabling hotplug
> + * handling would mean waiting on fb_helper.
> + */
> +bool
> +drm_fb_helper_suspend_hotplug(struct drm_fb_helper *fb_helper)
> +{
> + int ret;
> +
> + ret = mutex_trylock(&fb_helper->lock);
> + if (!ret)
> + return false;
> +
> + fb_helper->hotplug_suspended = true;
> + mutex_unlock(&fb_helper->lock);
> +
> + return true;
> +}
> +EXPORT_SYMBOL(drm_fb_helper_suspend_hotplug);
> +
> /**
> * drm_fb_helper_hotplug_event - respond to a hotplug notification by
> * probing all the outputs attached to the fb
> @@ -2774,6 +2834,11 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
> return err;
> }
>
> + if (fb_helper->hotplug_suspended) {
> + mutex_unlock(&fb_helper->lock);
> + return err;
> + }
> +
> DRM_DEBUG_KMS("\n");
>
> drm_setup_crtcs(fb_helper, fb_helper->fb->width, fb_helper->fb->height);
> diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
> index b069433e7fc1..30881131075c 100644
> --- a/include/drm/drm_fb_helper.h
> +++ b/include/drm/drm_fb_helper.h
> @@ -232,6 +232,14 @@ struct drm_fb_helper {
> * See also: @deferred_setup
> */
> int preferred_bpp;
> +
> + /**
> + * @hotplug_suspended:
> + *
> + * Whether or not we can currently handle hotplug events, or if we
> + * need to wait for the DRM device to uninhibit us.
> + */
> + bool hotplug_suspended;
> };
>
> /**
> @@ -330,6 +338,10 @@ void drm_fb_helper_fbdev_teardown(struct drm_device *dev);
>
> void drm_fb_helper_lastclose(struct drm_device *dev);
> void drm_fb_helper_output_poll_changed(struct drm_device *dev);
> +
> +void drm_fb_helper_resume_hotplug(struct drm_fb_helper *fb_helper);
> +bool drm_fb_helper_suspend_hotplug(struct drm_fb_helper *fb_helper);
> +
> #else
> static inline void drm_fb_helper_prepare(struct drm_device *dev,
> struct drm_fb_helper *helper,
> @@ -564,6 +576,14 @@ static inline void drm_fb_helper_output_poll_changed(struct drm_device *dev)
> {
> }
>
> +static inline void
> +drm_fb_helper_resume_hotplug(struct drm_fb_helper *fb_helper)
> +{
> +}
> +static inline bool
> +drm_fb_helper_suspend_hotplug(struct drm_fb_helper *fb_helper)
> +{
> +}
> #endif
>
> static inline int
> --
> 2.17.1
>
> _______________________________________________
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> dri-devel@xxxxxxxxxxxxxxxxxxxxx
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch