From cd06ab2fd48f2c0243b06344a36056e811d263b8 Mon Sep 17 00:00:00 2001 From: Jani Nikula Date: Fri, 1 Oct 2021 12:14:44 +0300 Subject: 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 Cc: Dave Airlie Reviewed-by: Daniel Vetter Signed-off-by: Jani Nikula Link: https://patchwork.freedesktop.org/patch/msgid/20211001091444.8177-1-jani.nikula@intel.com --- include/drm/drm_modeset_lock.h | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'include/drm') diff --git a/include/drm/drm_modeset_lock.h b/include/drm/drm_modeset_lock.h index aafd07388eb7..b84693fbd2b5 100644 --- a/include/drm/drm_modeset_lock.h +++ b/include/drm/drm_modeset_lock.h @@ -24,6 +24,8 @@ #ifndef DRM_MODESET_LOCK_H_ #define DRM_MODESET_LOCK_H_ +#include /* stackdepot.h is not self-contained */ +#include #include struct drm_modeset_lock; @@ -51,6 +53,12 @@ struct drm_modeset_acquire_ctx { */ struct drm_modeset_lock *contended; + /* + * Stack depot for debugging when a contended lock was not backed off + * from. + */ + depot_stack_handle_t stack_depot; + /* * list of held locks (drm_modeset_lock) */ -- cgit v1.2.3