drm/locking: add backtrace for locking contended locks without backoff
If drm_modeset_lock() returns -EDEADLK, the caller is supposed to drop all currently held locks using drm_modeset_backoff(). Failing to do so will result in warnings and backtraces on the paths trying to lock a contended lock. Add support for optionally printing the backtrace on the path that hit the deadlock and didn't gracefully handle the situation. For example, the patch [1] inadvertently dropped the return value check and error return on replacing calc_watermark_data() with intel_compute_global_watermarks(). The backtraces on the subsequent locking paths hitting WARN_ON(ctx->contended) were unhelpful, but adding the backtrace to the deadlock path produced this helpful printout: <7> [98.002465] drm_modeset_lock attempting to lock a contended lock without backoff: drm_modeset_lock+0x107/0x130 drm_atomic_get_plane_state+0x76/0x150 skl_compute_wm+0x251d/0x2b20 [i915] intel_atomic_check+0x1942/0x29e0 [i915] drm_atomic_check_only+0x554/0x910 drm_atomic_nonblocking_commit+0xe/0x50 drm_mode_atomic_ioctl+0x8c2/0xab0 drm_ioctl_kernel+0xac/0x140 Add new CONFIG_DRM_DEBUG_MODESET_LOCK to enable modeset lock debugging with stack depot and trace. [1] https://lore.kernel.org/r/20210924114741.15940-4-jani.nikula@intel.com v2: - default y if DEBUG_WW_MUTEX_SLOWPATH (Daniel) - depends on DEBUG_KERNEL Cc: Daniel Vetter <daniel@ffwll.ch> Cc: Dave Airlie <airlied@gmail.com> Reviewed-by:Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by:
Jani Nikula <jani.nikula@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20211001091444.8177-1-jani.nikula@intel.com
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