[Intel-gfx] [PATCH] drm/atomic: Add WARN_ON when state->acquire_ctx is not set.

[Daniel Vetter]

On Tue, May 03, 2016 at 11:12:31AM +0200, Maarten Lankhorst wrote:
> When I was writing an atomic wrapper for rmfb, I ran into the
> following backtrace from lockdep:
> 
> =============================================
> [ INFO: possible recursive locking detected ]
> 4.5.0-patser+ #4696 Tainted: G     U
> ---------------------------------------------
> kworker/2:2/2608 is trying to acquire lock:
>  (crtc_ww_class_mutex){+.+.+.}, at: [<ffffffffc00c9ddc>] drm_modeset_lock+0x7c/0x120 [drm]
> 
> but task is already holding lock:
>  (crtc_ww_class_mutex){+.+.+.}, at: [<ffffffffc00c98cd>] modeset_backoff+0x8d/0x220 [drm]
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(crtc_ww_class_mutex);
>   lock(crtc_ww_class_mutex);
> 
>  *** DEADLOCK ***
> 
>  May be due to missing lock nesting notation
> 
> 4 locks held by kworker/2:2/2608:
>  #0:  ("events"){.+.+.+}, at: [<ffffffff810a5eea>] process_one_work+0x15a/0x6c0
>  #1:  ((&arg.work)){+.+.+.}, at: [<ffffffff810a5eea>] process_one_work+0x15a/0x6c0
>  #2:  (crtc_ww_class_acquire){+.+.+.}, at: [<ffffffffc004532a>] drm_atomic_helper_remove_fb+0x4a/0x1d0 [drm_kms_helper]
>  #3:  (crtc_ww_class_mutex){+.+.+.}, at: [<ffffffffc00c98cd>] modeset_backoff+0x8d/0x220 [drm]
> 
> While lockdep probably catches this bug when it happens, it's better
> to explicitly warn when state->acquire_ctx is not set.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>

Yeah, because of the automagic fallback to normal mutex semantics when
acquire_ctx == NULL in ww_mutex_lock this can go boom, and the lockdep
splat is pretty hard to read. Makes sense to have an explicit WARN_ON
about it.

Applied to drm-misc, thanks.
-Daniel

> ---
>  drivers/gpu/drm/drm_atomic.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index 0b526423f19f..69adcf3944cc 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -263,6 +263,8 @@ drm_atomic_get_crtc_state(struct drm_atomic_state *state,
>  	int ret, index = drm_crtc_index(crtc);
>  	struct drm_crtc_state *crtc_state;
>  
> +	WARN_ON(!state->acquire_ctx);
> +
>  	crtc_state = drm_atomic_get_existing_crtc_state(state, crtc);
>  	if (crtc_state)
>  		return crtc_state;
> @@ -622,6 +624,8 @@ drm_atomic_get_plane_state(struct drm_atomic_state *state,
>  	int ret, index = drm_plane_index(plane);
>  	struct drm_plane_state *plane_state;
>  
> +	WARN_ON(!state->acquire_ctx);
> +
>  	plane_state = drm_atomic_get_existing_plane_state(state, plane);
>  	if (plane_state)
>  		return plane_state;
> @@ -890,6 +894,8 @@ drm_atomic_get_connector_state(struct drm_atomic_state *state,
>  	struct drm_mode_config *config = &connector->dev->mode_config;
>  	struct drm_connector_state *connector_state;
>  
> +	WARN_ON(!state->acquire_ctx);
> +
>  	ret = drm_modeset_lock(&config->connection_mutex, state->acquire_ctx);
>  	if (ret)
>  		return ERR_PTR(ret);
> -- 
> 2.5.5
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch