Re: [PATCH 02/27] drm/i915/guc: Fix outstanding G2H accounting

From: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
To: Matthew Brost <matthew.brost@intel.com>,
	<intel-gfx@lists.freedesktop.org>,
	<dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH 02/27] drm/i915/guc: Fix outstanding G2H accounting
Date: Thu, 26 Aug 2021 16:09:59 -0700	[thread overview]
Message-ID: <3fde98f9-b408-63cf-ea31-779bf477b68e@intel.com> (raw)
In-Reply-To: <20210826032327.18078-3-matthew.brost@intel.com>

On 8/25/2021 8:23 PM, Matthew Brost wrote:
> A small race that could result in incorrect accounting of the number
> of outstanding G2H. Basically prior to this patch we did not increment
> the number of outstanding G2H if we encoutered a GT reset while sending
> a H2G. This was incorrect as the context state had already been updated
> to anticipate a G2H response thus the counter should be incremented.
>
> Also always use helper when decrementing this value.
>
> Fixes: f4eb1f3fe946 ("drm/i915/guc: Ensure G2H response has space in buffer")
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Cc: <stable@vger.kernel.org>
> ---
>   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 23 ++++++++++---------
>   1 file changed, 12 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 69faa39da178..03a86da6011e 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -352,6 +352,12 @@ static inline void set_lrc_desc_registered(struct intel_guc *guc, u32 id,
>   	xa_unlock_irqrestore(&guc->context_lookup, flags);
>   }
>   
> +static void decr_outstanding_submission_g2h(struct intel_guc *guc)
> +{
> +	if (atomic_dec_and_test(&guc->outstanding_submission_g2h))
> +		wake_up_all(&guc->ct.wq);
> +}
> +
>   static int guc_submission_send_busy_loop(struct intel_guc *guc,
>   					 const u32 *action,
>   					 u32 len,
> @@ -360,11 +366,12 @@ static int guc_submission_send_busy_loop(struct intel_guc *guc,
>   {
>   	int err;
>   
> -	err = intel_guc_send_busy_loop(guc, action, len, g2h_len_dw, loop);
> -
> -	if (!err && g2h_len_dw)
> +	if (g2h_len_dw)
>   		atomic_inc(&guc->outstanding_submission_g2h);
>   
> +	err = intel_guc_send_busy_loop(guc, action, len, g2h_len_dw, loop);
> +	GEM_BUG_ON(g2h_len_dw && err == -EBUSY);

AFAICS having a return g2h is not tied to not returning EBUSY, the only 
way to avoid  EBUSY seems to be for loop to be true. maybe have instead:

GEM_BUG_ON(g2h_len_dw && !loop);

earlier on?

Daniele

> +
>   	return err;
>   }
>   
> @@ -616,7 +623,7 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
>   		init_sched_state(ce);
>   
>   		if (pending_enable || destroyed || deregister) {
> -			atomic_dec(&guc->outstanding_submission_g2h);
> +			decr_outstanding_submission_g2h(guc);
>   			if (deregister)
>   				guc_signal_context_fence(ce);
>   			if (destroyed) {
> @@ -635,7 +642,7 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
>   				intel_engine_signal_breadcrumbs(ce->engine);
>   			}
>   			intel_context_sched_disable_unpin(ce);
> -			atomic_dec(&guc->outstanding_submission_g2h);
> +			decr_outstanding_submission_g2h(guc);
>   			spin_lock_irqsave(&ce->guc_state.lock, flags);
>   			guc_blocked_fence_complete(ce);
>   			spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> @@ -2583,12 +2590,6 @@ g2h_context_lookup(struct intel_guc *guc, u32 desc_idx)
>   	return ce;
>   }
>   
> -static void decr_outstanding_submission_g2h(struct intel_guc *guc)
> -{
> -	if (atomic_dec_and_test(&guc->outstanding_submission_g2h))
> -		wake_up_all(&guc->ct.wq);
> -}
> -
>   int intel_guc_deregister_done_process_msg(struct intel_guc *guc,
>   					  const u32 *msg,
>   					  u32 len)