Re: [Intel-gfx] [PATCH 14/27] drm/i915/guc: Don't touch guc_state.sched_state without a lock

[Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>]

On 8/18/2021 11:16 PM, Matthew Brost wrote:
> Before we did some clever tricks to not use the a lock when touching
> guc_state.sched_state in certain cases. Don't do that, enforce the use
> of the lock.
>
> Part of this is removing a dead code path from guc_lrc_desc_pin where a
> context could be deregistered when the aforementioned function was
> called from the submission path. Remove this dead code and add a
> GEM_BUG_ON if this path is ever attempted to be used.
>
> v2:
>   (kernel test robo )
>    - Add __maybe_unused to sched_state_is_init()
>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Reported-by: kernel test robot <lkp@intel.com>
> ---
>   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 58 ++++++++++---------
>   1 file changed, 32 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index 85f96d325048..fa87470ea576 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -150,11 +150,23 @@ static inline void clr_context_registered(struct intel_context *ce)
>   #define SCHED_STATE_BLOCKED_MASK	(0xfff << SCHED_STATE_BLOCKED_SHIFT)
>   static inline void init_sched_state(struct intel_context *ce)
>   {
> -	/* Only should be called from guc_lrc_desc_pin() */
> +	lockdep_assert_held(&ce->guc_state.lock);
>   	atomic_set(&ce->guc_sched_state_no_lock, 0);
>   	ce->guc_state.sched_state &= SCHED_STATE_BLOCKED_MASK;
>   }
>   
> +__maybe_unused
> +static bool sched_state_is_init(struct intel_context *ce)
> +{
> +	/*
> +	 * XXX: Kernel contexts can have SCHED_STATE_NO_LOCK_REGISTERED after
> +	 * suspend.
> +	 */

This seems like something we want to fix. Not a blocker for this, but we 
can add it to the list.

> +	return !(atomic_read(&ce->guc_sched_state_no_lock) &
> +		 ~SCHED_STATE_NO_LOCK_REGISTERED) &&
> +		!(ce->guc_state.sched_state &= ~SCHED_STATE_BLOCKED_MASK);
> +}
> +
>   static inline bool
>   context_wait_for_deregister_to_register(struct intel_context *ce)
>   {
> @@ -165,7 +177,7 @@ context_wait_for_deregister_to_register(struct intel_context *ce)
>   static inline void
>   set_context_wait_for_deregister_to_register(struct intel_context *ce)
>   {
> -	/* Only should be called from guc_lrc_desc_pin() without lock */
> +	lockdep_assert_held(&ce->guc_state.lock);
>   	ce->guc_state.sched_state |=
>   		SCHED_STATE_WAIT_FOR_DEREGISTER_TO_REGISTER;
>   }
> @@ -605,9 +617,7 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
>   	bool pending_disable, pending_enable, deregister, destroyed, banned;
>   
>   	xa_for_each(&guc->context_lookup, index, ce) {
> -		/* Flush context */
>   		spin_lock_irqsave(&ce->guc_state.lock, flags);
> -		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
>   
>   		/*
>   		 * Once we are at this point submission_disabled() is guaranteed
> @@ -623,6 +633,8 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
>   		banned = context_banned(ce);
>   		init_sched_state(ce);
>   
> +		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> +
>   		if (pending_enable || destroyed || deregister) {
>   			decr_outstanding_submission_g2h(guc);
>   			if (deregister)
> @@ -1325,6 +1337,7 @@ static int guc_lrc_desc_pin(struct intel_context *ce, bool loop)
>   	int ret = 0;
>   
>   	GEM_BUG_ON(!engine->mask);
> +	GEM_BUG_ON(!sched_state_is_init(ce));
>   
>   	/*
>   	 * Ensure LRC + CT vmas are is same region as write barrier is done
> @@ -1353,7 +1366,6 @@ static int guc_lrc_desc_pin(struct intel_context *ce, bool loop)
>   	desc->priority = ce->guc_prio;
>   	desc->context_flags = CONTEXT_REGISTRATION_FLAG_KMD;
>   	guc_context_policy_init(engine, desc);
> -	init_sched_state(ce);
>   
>   	/*
>   	 * The context_lookup xarray is used to determine if the hardware
> @@ -1364,26 +1376,23 @@ static int guc_lrc_desc_pin(struct intel_context *ce, bool loop)
>   	 * registering this context.
>   	 */
>   	if (context_registered) {
> +		bool disabled;
> +		unsigned long flags;
> +
>   		trace_intel_context_steal_guc_id(ce);
> -		if (!loop) {
> +		GEM_BUG_ON(!loop);
> +
> +		/* Seal race with Reset */
> +		spin_lock_irqsave(&ce->guc_state.lock, flags);
> +		disabled = submission_disabled(guc);
> +		if (likely(!disabled)) {
>   			set_context_wait_for_deregister_to_register(ce);
>   			intel_context_get(ce);
> -		} else {
> -			bool disabled;
> -			unsigned long flags;
> -
> -			/* Seal race with Reset */
> -			spin_lock_irqsave(&ce->guc_state.lock, flags);
> -			disabled = submission_disabled(guc);
> -			if (likely(!disabled)) {
> -				set_context_wait_for_deregister_to_register(ce);
> -				intel_context_get(ce);
> -			}
> -			spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> -			if (unlikely(disabled)) {
> -				reset_lrc_desc(guc, desc_idx);
> -				return 0;	/* Will get registered later */
> -			}
> +		}
> +		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> +		if (unlikely(disabled)) {
> +			reset_lrc_desc(guc, desc_idx);
> +			return 0;	/* Will get registered later */
>   		}
>   
>   		/*
> @@ -1392,10 +1401,7 @@ static int guc_lrc_desc_pin(struct intel_context *ce, bool loop)
>   		 */
>   		with_intel_runtime_pm(runtime_pm, wakeref)
>   			ret = deregister_context(ce, ce->guc_id, loop);
> -		if (unlikely(ret == -EBUSY)) {
> -			clr_context_wait_for_deregister_to_register(ce);
> -			intel_context_put(ce);

Why is the EBUSY case not applicable anymore?

Daniele

> -		} else if (unlikely(ret == -ENODEV)) {
> +		if (unlikely(ret == -ENODEV)) {
>   			ret = 0;	/* Will get registered later */
>   		}
>   	} else {