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.nik...@intel.com
>
> Cc: Daniel Vetter <dan...@ffwll.ch>
> Cc: Dave Airlie <airl...@gmail.com>
> Signed-off-by: Jani Nikula <jani.nik...@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.
Either way:
Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>
> ---
> 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
