[PATCH] drm/locking: add backtrace for locking contended locks without backoff
Jani Nikula
jani.nikula at intel.com
Tue Sep 28 22:32:41 UTC 2021
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 at ffwll.ch>
Cc: Dave Airlie <airlied at gmail.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
---
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
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