drm/atomic: Paper over locking WARN in default_state_clear

This change “drm/atomic: Paper over locking WARN in default_state_clear” in Linux kernel is authored by Daniel Vetter <daniel.vetter [at] ffwll.ch> on Wed Jul 29 12:51:41 2015 +0200.

drm/atomic: Paper over locking WARN in default_state_clear

In

commit 6f75cea66c8dd043ced282016b21a639af176642
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Nov 19 18:38:07 2014 +0100

    drm/atomic: Only destroy connector states with connection mutex held

I tried to fix races of atomic commits against connector
hot-unplugging. The idea is to ensure lifetimes by holding the
connection_mutex long enough. That works for synchronous commits, but
not for async ones.

For async atomic commit we really need to fix up connector lifetimes
for real. But that's a much bigger task, so just add more duct-tape:
For cleaning up connector states we currently don't need the connector
itself. So NULL it out and remove the locking check. Of course that
check was to protect the entire sequence, but the modeset itself
should be save since currently DP MST hot-removal does a dpms-off. And
that should synchronize with any outstanding async atomic commit.

Or at least that's my hope, this is all a giant mess.

Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 460e8e2.

There are 12 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/gpu/drm/drm_atomic.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 3efd91c..4349154 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -153,9 +153,15 @@ void drm_atomic_state_default_clear(struct drm_atomic_state *state)
 		if (!connector)
 			continue;
 
-		WARN_ON(!drm_modeset_is_locked(&config->connection_mutex));
-
-		connector->funcs->atomic_destroy_state(connector,
+		/*
+		 * FIXME: Async commits can race with connector unplugging and
+		 * there's currently nothing that prevents cleanup up state for
+		 * deleted connectors. As long as the callback doesn't look at
+		 * the connector we'll be fine though, so make sure that's the
+		 * case by setting all connector pointers to NULL.
+		 */
+		state->connector_states[i]->connector = NULL;
+		connector->funcs->atomic_destroy_state(NULL,
 						       state->connector_states[i]);
 		state->connectors[i] = NULL;
 		state->connector_states[i] = NULL;

The commit for this change in Linux stable tree is 460e8e2 (patch).

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