On Wed, Sep 29, 2021 at 01:32:41AM +0300, Jani Nikula wrote: > 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@xxxxxxxxx > > Cc: Daniel Vetter <daniel@xxxxxxxx> > Cc: Dave Airlie <airlied@xxxxxxxxx> > Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx> I wonder whether we shouldn't just enable this when lock debugging is enabled? Otherwise we need to make sure CI have this set or it's not very useful. Or at least a default y if CONFIG_DEBUG_WW_MUTEX_SLOWPATH or something like that. Either way: Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx> > --- > drivers/gpu/drm/Kconfig | 13 ++++++++ > drivers/gpu/drm/drm_modeset_lock.c | 49 ++++++++++++++++++++++++++++-- > include/drm/drm_modeset_lock.h | 8 +++++ > 3 files changed, 68 insertions(+), 2 deletions(-) > > diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig > index b17e231ca6f7..7334975c788b 100644 > --- a/drivers/gpu/drm/Kconfig > +++ b/drivers/gpu/drm/Kconfig > @@ -100,6 +100,19 @@ 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 > + select STACKDEPOT > + depends on EXPERT > + 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) > */ > -- > 2.30.2 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
