On Thu, 30 Sep 2021, Daniel Vetter <daniel@xxxxxxxx> wrote: > 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. Added the conditional default y, as well as depends on DEBUG_KERNEL. > > Either way: > > Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx> Thanks, Jani. > >> --- >> 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 >> -- Jani Nikula, Intel Open Source Graphics Center
