[PATCH] drm: Drop modeset locking from crtc init function

[Daniel Vetter]

At driver init no one can access modeset objects and we're
single-threaded. So locking is just cargo-culting here. Worse, with
the new ww mutexes and ww mutex slowpath debugging the mutex_lock
might actually fail, and we don't have the full-blown ww recovery
dance.

Which then leads to fireworks when we try to unlock the not-locked
crtc lock.

An audit of all the functions called from here shows that none of them
contain locking checks, so there's also no reason to keep the locking
around just for consistency of caller contexts. Besides that I have
the rule (at least in i915) that such places where we take locks just
to simplify locking checks and not for correctness always require a
comment.

This regression was introduced in

commit 51fd371bbaf94018a1223b4e2cf20b9880fd92d4
Author: Rob Clark <robdclark at gmail.com>
Date:   Tue Nov 19 12:10:12 2013 -0500

    drm: convert crtc and connection_mutex to ww_mutex (v5)

v2: Don't drop the lock_init call, spotted by the 0day builder.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=83341
Cc: Rob Clark <robdclark at gmail.com>
Cc: thellstrom at vmware.com
Cc: maarten.lankhorst at canonical.com
Cc: stable at vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/drm_crtc.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index e2f4b8c21440..b1271a8d8ce7 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -679,11 +679,7 @@ int drm_crtc_init_with_planes(struct drm_device *dev, struct drm_crtc *crtc,
 	crtc->funcs = funcs;
 	crtc->invert_dimensions = false;
 
-	drm_modeset_lock_all(dev);
 	drm_modeset_lock_init(&crtc->mutex);
-	/* dropped by _unlock_all(): */
-	drm_modeset_lock(&crtc->mutex, config->acquire_ctx);
-
 	ret = drm_mode_object_get(dev, &crtc->base, DRM_MODE_OBJECT_CRTC);
 	if (ret)
 		goto out;
@@ -701,7 +697,6 @@ int drm_crtc_init_with_planes(struct drm_device *dev, struct drm_crtc *crtc,
 		cursor->possible_crtcs = 1 << drm_crtc_index(crtc);
 
  out:
-	drm_modeset_unlock_all(dev);
 
 	return ret;
 }
-- 
2.0.1