drm/nouveau: fix vblank deadlock

This fixes a deadlock inversion when vblank is enabled/disabled by drm. &dev->vblank_time_lock is always taken when the vblank state is toggled, which caused a deadlock when &event->lock was also taken during event_get/put.

Solve the race by requiring that lock to change enable/disable state, and always keeping vblank on the event list. Core drm ignores unwanted vblanks, so extra calls to drm_handle_vblank are harmless.

Signed-off-by Maarten Lankhorst <[email protected]>

---

drivers/gpu/drm/nouveau/core/core/event.c +51 -21

index 7eb81c1..78bff7c 100644
@@ -23,43 +23,64 @@
  #include <core/os.h>
  #include <core/event.h>
 
-static void
-nouveau_event_put_locked(struct nouveau_event *event, int index,
-			 struct nouveau_eventh *handler)
-{
-	if (!--event->index[index].refs) {
-		if (event->disable)
-			event->disable(event, index);
-	}
-	list_del(&handler->head);
-}
-
  void
  nouveau_event_put(struct nouveau_event *event, int index,
  		  struct nouveau_eventh *handler)
  {
  	unsigned long flags;
 
+	if (index >= event->index_nr)
+		return;
+
  	spin_lock_irqsave(&event->lock, flags);
-	if (index < event->index_nr)
-		nouveau_event_put_locked(event, index, handler);
+	list_del(&handler->head);
+
+	if (event->toggle_lock)
+		spin_lock(event->toggle_lock);
+	nouveau_event_disable_locked(event, index, 1);
+	if (event->toggle_lock)
+		spin_unlock(event->toggle_lock);
+
  	spin_unlock_irqrestore(&event->lock, flags);
  }
 
  void
+nouveau_event_enable_locked(struct nouveau_event *event, int index)
+{
+	if (index >= event->index_nr)
+		return;
+
+	if (!event->index[index].refs++ && event->enable)
+		event->enable(event, index);
+}
+
+void
+nouveau_event_disable_locked(struct nouveau_event *event, int index, int refs)
+{
+	if (index >= event->index_nr)
+		return;
+
+	event->index[index].refs -= refs;
+	if (!event->index[index].refs && event->disable)
+		event->disable(event, index);
+}
+
+void
  nouveau_event_get(struct nouveau_event *event, int index,
  		  struct nouveau_eventh *handler)
  {
  	unsigned long flags;
 
+	if (index >= event->index_nr)
+		return;
+
  	spin_lock_irqsave(&event->lock, flags);
-	if (index < event->index_nr) {
-		list_add(&handler->head, &event->index[index].list);
-		if (!event->index[index].refs++) {
-			if (event->enable)
-				event->enable(event, index);
-		}
-	}
+	list_add(&handler->head, &event->index[index].list);
+	if (event->toggle_lock)
+		spin_lock(event->toggle_lock);
+	nouveau_event_enable_locked(event, index);
+	if (event->toggle_lock)
+		spin_unlock(event->toggle_lock);
  	spin_unlock_irqrestore(&event->lock, flags);
  }
 

@@ -68,6 +89,7 @@ nouveau_event_trigger(struct nouveau_event *event, int index)
  {
  	struct nouveau_eventh *handler, *temp;
  	unsigned long flags;
+	int refs = 0;
 
  	if (index >= event->index_nr)
  		return;

@@ -75,9 +97,17 @@ nouveau_event_trigger(struct nouveau_event *event, int index)
  	spin_lock_irqsave(&event->lock, flags);
  	list_for_each_entry_safe(handler, temp, &event->index[index].list, head) {
  		if (handler->func(handler, index) == NVKM_EVENT_DROP) {
-			nouveau_event_put_locked(event, index, handler);
+			list_del(&handler->head);
+			refs++;
  		}
  	}
+	if (refs) {
+		if (event->toggle_lock)
+			spin_lock(event->toggle_lock);
+		nouveau_event_disable_locked(event, index, refs);
+		if (event->toggle_lock)
+			spin_unlock(event->toggle_lock);
+	}
  	spin_unlock_irqrestore(&event->lock, flags);
  }
 

drivers/gpu/drm/nouveau/core/include/core/event.h +5 -0

index 9e09440..b1a6c91 100644
@@ -12,6 +12,7 @@ struct nouveau_eventh {
 
  struct nouveau_event {
  	spinlock_t lock;
+	spinlock_t *toggle_lock;
 
  	void *priv;
  	void (*enable)(struct nouveau_event *, int index);

@@ -33,4 +34,8 @@ void nouveau_event_get(struct nouveau_event *, int index,
  void nouveau_event_put(struct nouveau_event *, int index,
  		       struct nouveau_eventh *);
 
+void nouveau_event_enable_locked(struct nouveau_event *event, int index);
+void nouveau_event_disable_locked(struct nouveau_event *event, int index,
+				  int refs);
+
  #endif

drivers/gpu/drm/nouveau/nouveau_crtc.h +1 -0

index d1e5890..398baa3 100644
@@ -29,6 +29,7 @@
 
  struct nouveau_crtc {
  	struct drm_crtc base;
+	struct nouveau_eventh vblank;
 
  	int index;
 

drivers/gpu/drm/nouveau/nouveau_display.h +1 -0

index 1ea3e47..4ba8cb5 100644
@@ -72,6 +72,7 @@ int  nouveau_display_dumb_destroy(struct drm_file *, struct drm_device *,
  				  u32 handle);
 
  void nouveau_hdmi_mode_set(struct drm_encoder *, struct drm_display_mode *);
+int nouveau_drm_vblank_handler(struct nouveau_eventh *event, int head);
 
  #ifdef CONFIG_DRM_NOUVEAU_BACKLIGHT
  extern int nouveau_backlight_init(struct drm_device *);

drivers/gpu/drm/nouveau/nouveau_drm.c +11 -13

index 4bf8dc3..bd301f4 100644
@@ -46,6 +46,7 @@
  #include "nouveau_pm.h"
  #include "nouveau_acpi.h"
  #include "nouveau_bios.h"
+#include "nouveau_crtc.h"
  #include "nouveau_ioctl.h"
  #include "nouveau_abi16.h"
  #include "nouveau_fbcon.h"

@@ -71,12 +72,13 @@ module_param_named(modeset, nouveau_modeset, int, 0400);
 
  static struct drm_driver driver;
 
-static int
+int
  nouveau_drm_vblank_handler(struct nouveau_eventh *event, int head)
  {
-	struct nouveau_drm *drm =
-		container_of(event, struct nouveau_drm, vblank[head]);
-	drm_handle_vblank(drm->dev, head);
+	struct nouveau_crtc *nv_crtc =
+		container_of(event, struct nouveau_crtc, vblank);
+
+	drm_handle_vblank(nv_crtc->base.dev, head);
  	return NVKM_EVENT_KEEP;
  }
 

@@ -86,11 +88,9 @@ nouveau_drm_vblank_enable(struct drm_device *dev, int head)
  	struct nouveau_drm *drm = nouveau_drm(dev);
  	struct nouveau_disp *pdisp = nouveau_disp(drm->device);
 
-	if (WARN_ON_ONCE(head > ARRAY_SIZE(drm->vblank)))
+	if (head >= pdisp->vblank->index_nr)
  		return -EIO;
-	WARN_ON_ONCE(drm->vblank[head].func);
-	drm->vblank[head].func = nouveau_drm_vblank_handler;
-	nouveau_event_get(pdisp->vblank, head, &drm->vblank[head]);
+	nouveau_event_enable_locked(pdisp->vblank, head);
  	return 0;
  }
 

@@ -99,11 +99,9 @@ nouveau_drm_vblank_disable(struct drm_device *dev, int head)
  {
  	struct nouveau_drm *drm = nouveau_drm(dev);
  	struct nouveau_disp *pdisp = nouveau_disp(drm->device);
-	if (drm->vblank[head].func)
-		nouveau_event_put(pdisp->vblank, head, &drm->vblank[head]);
-	else
-		WARN_ON_ONCE(1);
-	drm->vblank[head].func = NULL;
+
+	if (head < pdisp->vblank->index_nr)
+		nouveau_event_disable_locked(pdisp->vblank, head, 1);
  }
 
  static u64

drivers/gpu/drm/nouveau/nouveau_drm.h +0 -1

index 41ff7e0..0d0ba3b 100644
@@ -125,7 +125,6 @@ struct nouveau_drm {
  	struct nvbios vbios;
  	struct nouveau_display *display;
  	struct backlight_device *backlight;
-	struct nouveau_eventh vblank[4];
 
  	/* power management */
  	struct nouveau_pm *pm;

drivers/gpu/drm/nouveau/nv50_display.c +15 -0

index 007731d..738d7a2 100644
@@ -39,6 +39,7 @@
  #include <core/client.h>
  #include <core/gpuobj.h>
  #include <core/class.h>
+#include <engine/disp.h>
 
  #include <subdev/timer.h>
  #include <subdev/bar.h>

@@ -1280,15 +1281,21 @@ nv50_crtc_gamma_set(struct drm_crtc *crtc, u16 *r, u16 *g, u16 *b,
  static void
  nv50_crtc_destroy(struct drm_crtc *crtc)
  {
+	struct nouveau_event *event = nouveau_disp(nouveau_dev(crtc->dev))->vblank;
  	struct nouveau_crtc *nv_crtc = nouveau_crtc(crtc);
  	struct nv50_disp *disp = nv50_disp(crtc->dev);
  	struct nv50_head *head = nv50_head(crtc);
+	unsigned long flags;
 
  	nv50_dmac_destroy(disp->core, &head->ovly.base);
  	nv50_pioc_destroy(disp->core, &head->oimm.base);
  	nv50_dmac_destroy(disp->core, &head->sync.base);
  	nv50_pioc_destroy(disp->core, &head->curs.base);
 
+	spin_lock_irqsave(&event->lock, flags);
+	list_del(&nv_crtc->vblank.head);
+	spin_unlock_irqrestore(&event->lock, flags);
+
  	/*XXX: this shouldn't be necessary, but the core doesn't call
  	 *     disconnect() during the cleanup paths
  	 */

@@ -1344,10 +1351,12 @@ nv50_cursor_set_offset(struct nouveau_crtc *nv_crtc, uint32_t offset)
  static int
  nv50_crtc_create(struct drm_device *dev, struct nouveau_object *core, int index)
  {
+	struct nouveau_event *event = nouveau_disp(nouveau_dev(dev))->vblank;
  	struct nv50_disp *disp = nv50_disp(dev);
  	struct nv50_head *head;
  	struct drm_crtc *crtc;
  	int ret, i;
+	unsigned long flags;
 
  	head = kzalloc(sizeof(*head), GFP_KERNEL);
  	if (!head)

@@ -1372,6 +1381,12 @@ nv50_crtc_create(struct drm_device *dev, struct nouveau_object *core, int index)
  	drm_crtc_helper_add(crtc, &nv50_crtc_hfunc);
  	drm_mode_crtc_set_gamma_size(crtc, 256);
 
+	spin_lock_irqsave(&event->lock, flags);
+	event->toggle_lock = &dev->vblank_time_lock;
+	head->base.vblank.func = nouveau_drm_vblank_handler;
+	list_add(&head->base.vblank.head, &event->index[index].list);
+	spin_unlock_irqrestore(&event->lock, flags);
+
  	ret = nouveau_bo_new(dev, 8192, 0x100, TTM_PL_FLAG_VRAM,
  			     0, 0x0000, NULL, NULL, &head->base.lut.nvbo);
  	if (!ret) {

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Quoted 8 lines

This fixes a deadlock inversion when vblank is enabled/disabled by drm. &dev->vblank_time_lock is always taken when the vblank state is toggled, which caused a deadlock when &event->lock was also taken during event_get/put.

Solve the race by requiring that lock to change enable/disable state, and always keeping vblank on the event list. Core drm ignores unwanted vblanks, so extra calls to drm_handle_vblank are harmless.

I don't feel this is the appropriate solution to the lock inversion between vblank_time_lock and event->lock.

Preferably drm core should correct the interface layer bug; ie., calling into a sub-driver holding a lock _and_ requiring the sub-driver to call a drm helper function which claims the same lock is bad design. The console lock suffers from the same design flaw and is a constant problem.

Alternatively, the event trigger could be lockless; ie., the event list could be an RCU list instead. In this way, the event->lock does not need to be claimed, and thus no lock inversion is possible. The main drawback here is that currently the event->lock enforces non-overlapping lifetimes between the event handler and the event. Untangling object lifetimes in nouveau is a non-trivial exercise.