[PATCH] drm/fb-helper: Fix race between deferred_io worker and dirty updater

[Takashi Iwai]

Since 4.7 kernel, we've seen the error messages like

 kernel: [TTM] Buffer eviction failed
 kernel: qxl 0000:00:02.0: object_init failed for (4026540032, 0x00000001)
 kernel: [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to allocate VRAM BO

on QXL when switching and accessing on VT.  The culprit was the generic
deferred_io code (qxl driver switched to it since 4.7).  There is a
race between the dirty clip update and the call of callback.

In drm_fb_helper_dirty(), the dirty clip is updated in the spinlock,
while it kicks off the update worker outside the spinlock.  Meanwhile
the update worker clears the dirty clip in the spinlock, too.  Thus,
when drm_fb_helper_dirty() is called concurrently, schedule_work() is
called after the clip is cleared in the first worker call.

The fix is simply moving schedule_work() inside the spinlock.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98322
Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1003298
Fixes: eaa434defaca ('drm/fb-helper: Add fb_deferred_io support')
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 drivers/gpu/drm/drm_fb_helper.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 03414bde1f15..bae392dea2cc 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -861,9 +861,8 @@ static void drm_fb_helper_dirty(struct fb_info *info, u32 x, u32 y,
 	clip->y1 = min_t(u32, clip->y1, y);
 	clip->x2 = max_t(u32, clip->x2, x + width);
 	clip->y2 = max_t(u32, clip->y2, y + height);
-	spin_unlock_irqrestore(&helper->dirty_lock, flags);
-
 	schedule_work(&helper->dirty_work);
+	spin_unlock_irqrestore(&helper->dirty_lock, flags);
 }
 
 /**
-- 
2.10.1