Commit cd06ab2f authored by Jani Nikula's avatar Jani Nikula
Browse files

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 <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: default avatarJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001091444.8177-1-jani.nikula@intel.com
parent 91302d6c

drivers/gpu/drm/Kconfig

+15 −0
Original line number Diff line number Diff line
@@ -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

drivers/gpu/drm/drm_modeset_lock.c

+47 −2
Original line number Diff line number Diff line
@@ -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;

include/drm/drm_modeset_lock.h

+8 −0
Original line number Diff line number Diff line
@@ -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)

	 */