[PATCH] drm: use trylock to avoid fault injection antics

[Rob Clark]

While in real life, we could never fail to grab the newly created mutex,
ww_mutex fault injection has no way to know this.  Which could result
that kernels built with CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y might fail to
acquire the new crtc lock.  Which results in bad things when the locks
are dropped.

See: https://bugzilla.kernel.org/show_bug.cgi?id=83341

Signed-off-by: Rob Clark <robdclark at gmail.com>
---
 drivers/gpu/drm/drm_crtc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 7d7c1fd..8bb11fa 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -682,7 +682,15 @@ int drm_crtc_init_with_planes(struct drm_device *dev, struct drm_crtc *crtc,
 	drm_modeset_lock_all(dev);
 	drm_modeset_lock_init(&crtc->mutex);
 	/* dropped by _unlock_all(): */
-	drm_modeset_lock(&crtc->mutex, config->acquire_ctx);
+	/* NOTE: use trylock here for the benefit of ww_mutex fault
+	 * injection.  We cannot actually fail to grab this lock (as
+	 * it has only just been created), but fault injection does
+	 * not know this, which can result in the this lock failing,
+	 * and hilarity when we later try to drop the locks.  See:
+	 * https://bugzilla.kernel.org/show_bug.cgi?id=83341
+	 */
+	ret = ww_mutex_trylock(&crtc->mutex.mutex);
+	WARN_ON(ret);
 
 	ret = drm_mode_object_get(dev, &crtc->base, DRM_MODE_OBJECT_CRTC);
 	if (ret)
-- 
1.9.3