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 --- drivers/gpu/drm/Kconfig | 15 +++++++++ drivers/gpu/drm/drm_modeset_lock.c | 49 ++++++++++++++++++++++++++++-- include/drm/drm_modeset_lock.h | 8 +++++ 3 files changed, 70 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig index 2a926d0de423..a4c020a9a0eb 100644 --- a/drivers/gpu/drm/Kconfig +++ b/drivers/gpu/drm/Kconfig @@ -100,6 +100,21 @@ config DRM_DEBUG_DP_MST_TOPOLOGY_REFS This has the potential to use a lot of memory and print some very large kernel messages. If in doubt, say "N".
+config DRM_DEBUG_MODESET_LOCK + bool "Enable backtrace history for lock contention" + depends on STACKTRACE_SUPPORT + depends on DEBUG_KERNEL + depends on EXPERT + select STACKDEPOT + default y if DEBUG_WW_MUTEX_SLOWPATH + help + Enable debug tracing of failures to gracefully handle drm modeset lock + contention. A history of each drm modeset lock path hitting -EDEADLK + will be saved until gracefully handled, and the backtrace will be + printed when attempting to lock a contended lock. + + If in doubt, say "N". + config DRM_FBDEV_EMULATION bool "Enable legacy fbdev support for your modesetting driver" depends on DRM diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c index bf8a6e823a15..4d32b61fa1fd 100644 --- a/drivers/gpu/drm/drm_modeset_lock.c +++ b/drivers/gpu/drm/drm_modeset_lock.c @@ -25,6 +25,7 @@ #include <drm/drm_crtc.h> #include <drm/drm_device.h> #include <drm/drm_modeset_lock.h> +#include <drm/drm_print.h>
/** * DOC: kms locking @@ -77,6 +78,45 @@
static DEFINE_WW_CLASS(crtc_ww_class);
+#if IS_ENABLED(CONFIG_DRM_DEBUG_MODESET_LOCK) +static noinline depot_stack_handle_t __stack_depot_save(void) +{ + unsigned long entries[8]; + unsigned int n; + + n = stack_trace_save(entries, ARRAY_SIZE(entries), 1); + + return stack_depot_save(entries, n, GFP_NOWAIT | __GFP_NOWARN); +} + +static void __stack_depot_print(depot_stack_handle_t stack_depot) +{ + struct drm_printer p = drm_debug_printer("drm_modeset_lock"); + unsigned long *entries; + unsigned int nr_entries; + char *buf; + + buf = kmalloc(PAGE_SIZE, GFP_NOWAIT | __GFP_NOWARN); + if (!buf) + return; + + nr_entries = stack_depot_fetch(stack_depot, &entries); + stack_trace_snprint(buf, PAGE_SIZE, entries, nr_entries, 2); + + drm_printf(&p, "attempting to lock a contended lock without backoff:\n%s", buf); + + kfree(buf); +} +#else /* CONFIG_DRM_DEBUG_MODESET_LOCK */ +static depot_stack_handle_t __stack_depot_save(void) +{ + return 0; +} +static void __stack_depot_print(depot_stack_handle_t stack_depot) +{ +} +#endif /* CONFIG_DRM_DEBUG_MODESET_LOCK */ + /** * drm_modeset_lock_all - take all modeset locks * @dev: DRM device @@ -225,7 +265,9 @@ EXPORT_SYMBOL(drm_modeset_acquire_fini); */ void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx) { - WARN_ON(ctx->contended); + if (WARN_ON(ctx->contended)) + __stack_depot_print(ctx->stack_depot); + while (!list_empty(&ctx->locked)) { struct drm_modeset_lock *lock;
@@ -243,7 +285,8 @@ static inline int modeset_lock(struct drm_modeset_lock *lock, { int ret;
- WARN_ON(ctx->contended); + if (WARN_ON(ctx->contended)) + __stack_depot_print(ctx->stack_depot);
if (ctx->trylock_only) { lockdep_assert_held(&ctx->ww_ctx); @@ -274,6 +317,7 @@ static inline int modeset_lock(struct drm_modeset_lock *lock, ret = 0; } else if (ret == -EDEADLK) { ctx->contended = lock; + ctx->stack_depot = __stack_depot_save(); }
return ret; @@ -296,6 +340,7 @@ int drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx) struct drm_modeset_lock *contended = ctx->contended;
ctx->contended = NULL; + ctx->stack_depot = 0;
if (WARN_ON(!contended)) return 0; 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 <linux/types.h> /* stackdepot.h is not self-contained */ +#include <linux/stackdepot.h> #include <linux/ww_mutex.h>
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) */