[Intel-gfx] [PATCH v3 2/3] drm/i915/guc: Close deregister-context race against CT-loss

[Rodrigo Vivi]

On Sat, Sep 09, 2023 at 08:58:45PM -0700, Alan Previn wrote:
> If we are at the end of suspend or very early in resume
> its possible an async fence signal could lead us to the
> execution of the context destruction worker (after the
> prior worker flush).
> 
> Even if checking that the CT is enabled before calling
> destroyed_worker_func, guc_lrc_desc_unpin may still fail
> because in corner cases, as we traverse the GuC's
> context-destroy list, the CT could get disabled in the mid
> of it right before calling the GuC's CT send function.
> 
> We've witnessed this race condition once every ~6000-8000
> suspend-resume cycles while ensuring workloads that render
> something onscreen is continuously started just before
> we suspend (and the workload is small enough to complete
> and trigger the queued engine/context free-up either very
> late in suspend or very early in resume).
> 
> In such a case, we need to unroll the unpin process because
> guc-lrc-unpin takes a gt wakeref which only gets released in
> the G2H IRQ reply that never comes through in this corner
> case. That will cascade into a kernel hang later at the tail
> end of suspend in this function:
> 
>    intel_wakeref_wait_for_idle(&gt->wakeref)
>    (called by) - intel_gt_pm_wait_for_idle
>    (called by) - wait_for_suspend
> 
> Doing this unroll and keeping the context in the GuC's
> destroy-list will allow the context to get picked up on
> the next destroy worker invocation or purged as part of a
> major GuC sanitization or reset flow.
> 
> Signed-off-by: Alan Previn <alan.previn.teres.alexis at intel.com>
> Tested-by: Mousumi Jana <mousumi.jana at intel.com>
> ---
>  .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 76 ++++++++++++++++---
>  1 file changed, 65 insertions(+), 11 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 0ed44637bca0..db7df1217350 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -235,6 +235,13 @@ set_context_destroyed(struct intel_context *ce)
>  	ce->guc_state.sched_state |= SCHED_STATE_DESTROYED;
>  }
>  
> +static inline void
> +clr_context_destroyed(struct intel_context *ce)
> +{
> +	lockdep_assert_held(&ce->guc_state.lock);
> +	ce->guc_state.sched_state &= ~SCHED_STATE_DESTROYED;
> +}
> +
>  static inline bool context_pending_disable(struct intel_context *ce)
>  {
>  	return ce->guc_state.sched_state & SCHED_STATE_PENDING_DISABLE;
> @@ -612,6 +619,8 @@ static int guc_submission_send_busy_loop(struct intel_guc *guc,
>  					 u32 g2h_len_dw,
>  					 bool loop)
>  {
> +	int ret;
> +
>  	/*
>  	 * We always loop when a send requires a reply (i.e. g2h_len_dw > 0),
>  	 * so we don't handle the case where we don't get a reply because we
> @@ -622,7 +631,11 @@ static int guc_submission_send_busy_loop(struct intel_guc *guc,
>  	if (g2h_len_dw)
>  		atomic_inc(&guc->outstanding_submission_g2h);
>  
> -	return intel_guc_send_busy_loop(guc, action, len, g2h_len_dw, loop);
> +	ret = intel_guc_send_busy_loop(guc, action, len, g2h_len_dw, loop);
> +	if (ret)
> +		atomic_dec(&guc->outstanding_submission_g2h);
> +
> +	return ret;
>  }
>  
>  int intel_guc_wait_for_pending_msg(struct intel_guc *guc,
> @@ -3173,12 +3186,13 @@ static void guc_context_close(struct intel_context *ce)
>  	spin_unlock_irqrestore(&ce->guc_state.lock, flags);
>  }
>  
> -static inline void guc_lrc_desc_unpin(struct intel_context *ce)
> +static inline int guc_lrc_desc_unpin(struct intel_context *ce)
>  {
>  	struct intel_guc *guc = ce_to_guc(ce);
>  	struct intel_gt *gt = guc_to_gt(guc);
>  	unsigned long flags;
>  	bool disabled;
> +	int ret;
>  
>  	GEM_BUG_ON(!intel_gt_pm_is_awake(gt));
>  	GEM_BUG_ON(!ctx_id_mapped(guc, ce->guc_id.id));
> @@ -3188,19 +3202,33 @@ static inline void guc_lrc_desc_unpin(struct intel_context *ce)
>  	/* Seal race with Reset */
>  	spin_lock_irqsave(&ce->guc_state.lock, flags);
>  	disabled = submission_disabled(guc);
> -	if (likely(!disabled)) {
> -		__intel_gt_pm_get(gt);
> -		set_context_destroyed(ce);
> -		clr_context_registered(ce);
> -	}
> -	spin_unlock_irqrestore(&ce->guc_state.lock, flags);
>  	if (unlikely(disabled)) {
> +		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
>  		release_guc_id(guc, ce);
>  		__guc_context_destroy(ce);
> -		return;
> +		return 0;
>  	}
>  
> -	deregister_context(ce, ce->guc_id.id);
> +	/* GuC is active, lets destroy this context,
> +	 * but at this point we can still be racing with
> +	 * suspend, so we undo everything if the H2G fails
> +	 */
> +
> +	/* Change context state to destroyed and get gt-pm */
> +	__intel_gt_pm_get(gt);
> +	set_context_destroyed(ce);
> +	clr_context_registered(ce);
> +
> +	ret = deregister_context(ce, ce->guc_id.id);
> +	if (ret) {
> +		/* Undo the state change and put gt-pm if that failed */
> +		set_context_registered(ce);
> +		clr_context_destroyed(ce);
> +		intel_gt_pm_put(gt);

This is a might_sleep inside a spin_lock! Are you 100% confident no WARN
was seeing during the tests indicated in the commit msg?

> +	}
> +	spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> +
> +	return 0;

If you are always returning 0, there's no pointing in s/void/int...

>  }
>  
>  static void __guc_context_destroy(struct intel_context *ce)
> @@ -3268,7 +3296,22 @@ static void deregister_destroyed_contexts(struct intel_guc *guc)
>  		if (!ce)
>  			break;
>  
> -		guc_lrc_desc_unpin(ce);
> +		if (guc_lrc_desc_unpin(ce)) {
> +			/*
> +			 * This means GuC's CT link severed mid-way which could happen
> +			 * in suspend-resume corner cases. In this case, put the
> +			 * context back into the destroyed_contexts list which will
> +			 * get picked up on the next context deregistration event or
> +			 * purged in a GuC sanitization event (reset/unload/wedged/...).
> +			 */
> +			spin_lock_irqsave(&guc->submission_state.lock, flags);
> +			list_add_tail(&ce->destroyed_link,
> +				      &guc->submission_state.destroyed_contexts);
> +			spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> +			/* Bail now since the list might never be emptied if h2gs fail */
> +			break;
> +		}
> +
>  	}
>  }
>  
> @@ -3279,6 +3322,17 @@ static void destroyed_worker_func(struct work_struct *w)
>  	struct intel_gt *gt = guc_to_gt(guc);
>  	int tmp;
>  
> +	/*
> +	 * In rare cases we can get here via async context-free fence-signals that
> +	 * come very late in suspend flow or very early in resume flows. In these
> +	 * cases, GuC won't be ready but just skipping it here is fine as these
> +	 * pending-destroy-contexts get destroyed totally at GuC reset time at the
> +	 * end of suspend.. OR.. this worker can be picked up later on the next
> +	 * context destruction trigger after resume-completes

who is triggering the work queue again?
I mean, I'm wondering if it is necessary to re-schedule it from inside...
but also wonder if this is safe anyway...

> +	 */
> +	if (!intel_guc_is_ready(guc))
> +		return;
> +
>  	with_intel_gt_pm(gt, tmp)
>  		deregister_destroyed_contexts(guc);
>  }
> -- 
> 2.39.0
>