Re: [PATCH] drm/locking: add backtrace for locking contended locks without backoff

On Thu, 30 Sep 2021, Daniel Vetter daniel@ffwll.ch 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@intel.com

Cc: Daniel Vetter daniel@ffwll.ch Cc: Dave Airlie airlied@gmail.com Signed-off-by: Jani Nikula jani.nikula@intel.com

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@ffwll.ch

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