[PATCH] drm: Try to acquire modeset lock on panic or sysrq

[Thierry Reding]

From: Thierry Reding <treding at nvidia.com>

Commit 62ff94a54921 "drm/crtc-helper: remove LOCKING from kerneldoc"
causes drm_helper_crtc_in_use() and drm_helper_encoder_in_use() to
complain loudly during a kernel panic or sysrq processing. This is
caused by nobody acquiring the modeset lock in these code paths.

This patch fixes this by trying to acquire the modeset lock for each
FB helper that's forced to kernel mode. If the lock can't be acquired,
it's likely that somebody else is performing a modeset. However, doing
another modeset concurrently might make things even worse, so the safe
option is to simply bail out in that case.

Signed-off-by: Thierry Reding <treding at nvidia.com>
---
 drivers/gpu/drm/drm_fb_helper.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 9580393628a0..afce7cbd5149 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -325,12 +325,21 @@ static bool drm_fb_helper_force_kernel_mode(void)
 		return false;
 
 	list_for_each_entry(helper, &kernel_fb_helper_list, kernel_fb_list) {
-		if (helper->dev->switch_power_state == DRM_SWITCH_POWER_OFF)
+		struct drm_device *dev = helper->dev;
+
+		if (dev->switch_power_state == DRM_SWITCH_POWER_OFF)
+			continue;
+
+		if (!mutex_trylock(&dev->mode_config.mutex)) {
+			error = true;
 			continue;
+		}
 
 		ret = drm_fb_helper_restore_fbdev_mode(helper);
 		if (ret)
 			error = true;
+
+		mutex_unlock(&dev->mode_config.mutex);
 	}
 	return error;
 }
-- 
1.9.1