[PATCH] Prevent CPU deadlock with fbdev based consoles while printing scheduler warnings

[mpenttil at redhat.com]

From: Mika Penttilä <mpenttil at redhat.com>

With some fbdev based consoles, using the deferred_io mechanism and drm_fb_helper,
there can be a call chain like:

Backtrack:

try_to_wake_up  <-- rq_lock taken
__queue_work
queue_work_on
soft_cursor
hide_cursor
vt_console_print
console_unlock
vprintk_emit
printk
__warn_printk
(cfs_rq_is_decayed -> SCHED_WARN_ON)
__update_blocked_fair
update_blocked_averages   <-- rq_lock taken

Example producer is with qemu bochs virtio device (qemu stdvga),
and drm bochs support in the guest.

This can fixed be used using schedule_delayed_work() to get out of scheduler context,
if needed, while queueing the damage_work.

Signed-off-by: Mika Penttilä <mpenttil at redhat.com>
Cc: David Airlie <airlied at linux.ie>
---
 drivers/gpu/drm/drm_fb_helper.c | 8 ++++----
 include/drm/drm_fb_helper.h     | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 5ad2b6a2778c..6449e8dc97f6 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -429,7 +429,7 @@ static int drm_fb_helper_damage_blit(struct drm_fb_helper *fb_helper,
 static void drm_fb_helper_damage_work(struct work_struct *work)
 {
 	struct drm_fb_helper *helper = container_of(work, struct drm_fb_helper,
-						    damage_work);
+						    damage_work.work);
 	struct drm_device *dev = helper->dev;
 	struct drm_clip_rect *clip = &helper->damage_clip;
 	struct drm_clip_rect clip_copy;
@@ -488,7 +488,7 @@ void drm_fb_helper_prepare(struct drm_device *dev, struct drm_fb_helper *helper,
 	INIT_LIST_HEAD(&helper->kernel_fb_list);
 	spin_lock_init(&helper->damage_lock);
 	INIT_WORK(&helper->resume_work, drm_fb_helper_resume_worker);
-	INIT_WORK(&helper->damage_work, drm_fb_helper_damage_work);
+	INIT_DELAYED_WORK(&helper->damage_work, drm_fb_helper_damage_work);
 	helper->damage_clip.x1 = helper->damage_clip.y1 = ~0;
 	mutex_init(&helper->lock);
 	helper->funcs = funcs;
@@ -625,7 +625,7 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
 		return;
 
 	cancel_work_sync(&fb_helper->resume_work);
-	cancel_work_sync(&fb_helper->damage_work);
+	cancel_delayed_work_sync(&fb_helper->damage_work);
 
 	info = fb_helper->fbdev;
 	if (info) {
@@ -677,7 +677,7 @@ static void drm_fb_helper_damage(struct fb_info *info, u32 x, u32 y,
 	clip->y2 = max_t(u32, clip->y2, y + height);
 	spin_unlock_irqrestore(&helper->damage_lock, flags);
 
-	schedule_work(&helper->damage_work);
+	schedule_delayed_work(&helper->damage_work, in_atomic() ? 1 : 0);
 }
 
 /* Convert memory region into area of scanlines and pixels per scanline */
diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index 329607ca65c0..65a26d57d517 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -132,7 +132,7 @@ struct drm_fb_helper {
 	u32 pseudo_palette[17];
 	struct drm_clip_rect damage_clip;
 	spinlock_t damage_lock;
-	struct work_struct damage_work;
+	struct delayed_work damage_work;
 	struct work_struct resume_work;
 
 	/**
-- 
2.17.1