Re: [PATCH v5 2/8] drm/i915: Fix request locking during error capture & debugfs dump

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 26/01/2023 00:54, John.C.Harrison@xxxxxxxxx wrote:
> From: John Harrison <John.C.Harrison@xxxxxxxxx>
>
> When GuC support was added to error capture, the locking around the
> request object was broken. Fix it up.
>
> The context based search manages the spinlocking around the search
> internally. So it needs to grab the reference count internally as
> well. The execlist only request based search relies on external
> locking, so it needs an external reference count but within the
> spinlock not outside it.
>
> The only other caller of the context based search is the code for
> dumping engine state to debugfs. That code wasn't previously getting
> an explicit reference at all as it does everything while holding the
> execlist specific spinlock. So, that needs updaing as well as that
> spinlock doesn't help when using GuC submission. Rather than trying to
> conditionally get/put depending on submission model, just change it to
> always do the get/put.
>
> v2: Explicitly document adding an extra blank line in some dense code
> (Andy Shevchenko). Fix multiple potential null pointer derefs in case
> of no request found (some spotted by Tvrtko, but there was more!).
> Also fix a leaked request in case of !started and another in
> __guc_reset_context now that intel_context_find_active_request is
> actually reference counting the returned request.
> v3: Add a _get suffix to intel_context_find_active_request now that it
> grabs a reference (Daniele).
> v4: Split the intel_guc_find_hung_context change to a separate patch
> and rename intel_context_find_active_request_get to
> intel_context_get_active_request (Tvrtko).
>
> Fixes: dc0dad365c5e ("drm/i915/guc: Fix for error capture after full GPU reset with GuC")
> Fixes: 573ba126aef3 ("drm/i915/guc: Capture error state on context reset")
> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> Cc: John Harrison <John.C.Harrison@xxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>
> Cc: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
> Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@xxxxxxxxx>
> Cc: Michael Cheng <michael.cheng@xxxxxxxxx>
> Cc: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
> Cc: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay@xxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: Aravind Iddamsetty <aravind.iddamsetty@xxxxxxxxx>
> Cc: Alan Previn <alan.previn.teres.alexis@xxxxxxxxx>
> Cc: Bruce Chang <yu.bruce.chang@xxxxxxxxx>
> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
> Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>

Ideally if you could rename the patch to not say it is fixing locking 
but about reference counting, but perhaps that's too pedantic.

Series flow nicely for me now, thanks for reworking it. For the first 
three patches:

Acked-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko

> ---
>   drivers/gpu/drm/i915/gt/intel_context.c           |  4 +++-
>   drivers/gpu/drm/i915/gt/intel_context.h           |  3 +--
>   drivers/gpu/drm/i915/gt/intel_engine_cs.c         |  6 +++++-
>   drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c |  3 ++-
>   drivers/gpu/drm/i915/i915_gpu_error.c             | 13 ++++++-------
>   5 files changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index e94365b08f1ef..2aa63ec521b89 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -528,7 +528,7 @@ struct i915_request *intel_context_create_request(struct intel_context *ce)
>   	return rq;
>   }
>   
> -struct i915_request *intel_context_find_active_request(struct intel_context *ce)
> +struct i915_request *intel_context_get_active_request(struct intel_context *ce)
>   {
>   	struct intel_context *parent = intel_context_to_parent(ce);
>   	struct i915_request *rq, *active = NULL;
> @@ -552,6 +552,8 @@ struct i915_request *intel_context_find_active_request(struct intel_context *ce)
>   
>   		active = rq;
>   	}
> +	if (active)
> +		active = i915_request_get_rcu(active);
>   	spin_unlock_irqrestore(&parent->guc_state.lock, flags);
>   
>   	return active;
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
> index fb62b7b8cbcda..0a8d553da3f43 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> @@ -268,8 +268,7 @@ int intel_context_prepare_remote_request(struct intel_context *ce,
>   
>   struct i915_request *intel_context_create_request(struct intel_context *ce);
>   
> -struct i915_request *
> -intel_context_find_active_request(struct intel_context *ce);
> +struct i915_request *intel_context_get_active_request(struct intel_context *ce);
>   
>   static inline bool intel_context_is_barrier(const struct intel_context *ce)
>   {
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> index 922f1bb22dc68..a86bdbee7a6be 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> @@ -2237,9 +2237,11 @@ static void engine_dump_active_requests(struct intel_engine_cs *engine, struct d
>   	if (guc) {
>   		ce = intel_engine_get_hung_context(engine);
>   		if (ce)
> -			hung_rq = intel_context_find_active_request(ce);
> +			hung_rq = intel_context_get_active_request(ce);
>   	} else {
>   		hung_rq = intel_engine_execlist_find_hung_request(engine);
> +		if (hung_rq)
> +			hung_rq = i915_request_get_rcu(hung_rq);
>   	}
>   
>   	if (hung_rq)
> @@ -2250,6 +2252,8 @@ static void engine_dump_active_requests(struct intel_engine_cs *engine, struct d
>   	else
>   		intel_engine_dump_active_requests(&engine->sched_engine->requests,
>   						  hung_rq, m);
> +	if (hung_rq)
> +		i915_request_put(hung_rq);
>   }
>   
>   void intel_engine_dump(struct intel_engine_cs *engine,
> 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 3b34a82d692be..a2b263e5fd667 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -1702,7 +1702,7 @@ static void __guc_reset_context(struct intel_context *ce, intel_engine_mask_t st
>   			goto next_context;
>   
>   		guilty = false;
> -		rq = intel_context_find_active_request(ce);
> +		rq = intel_context_get_active_request(ce);
>   		if (!rq) {
>   			head = ce->ring->tail;
>   			goto out_replay;
> @@ -1715,6 +1715,7 @@ static void __guc_reset_context(struct intel_context *ce, intel_engine_mask_t st
>   		head = intel_ring_wrap(ce->ring, rq->head);
>   
>   		__i915_request_reset(rq, guilty);
> +		i915_request_put(rq);
>   out_replay:
>   		guc_reset_state(ce, head, guilty);
>   next_context:
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> index 9d5d5a397b64e..9e2d17785a9a8 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> @@ -1607,7 +1607,7 @@ capture_engine(struct intel_engine_cs *engine,
>   	ce = intel_engine_get_hung_context(engine);
>   	if (ce) {
>   		intel_engine_clear_hung_context(engine);
> -		rq = intel_context_find_active_request(ce);
> +		rq = intel_context_get_active_request(ce);
>   		if (!rq || !i915_request_started(rq))
>   			goto no_request_capture;
>   	} else {
> @@ -1618,21 +1618,18 @@ capture_engine(struct intel_engine_cs *engine,
>   		if (!intel_uc_uses_guc_submission(&engine->gt->uc)) {
>   			spin_lock_irqsave(&engine->sched_engine->lock, flags);
>   			rq = intel_engine_execlist_find_hung_request(engine);
> +			if (rq)
> +				rq = i915_request_get_rcu(rq);
>   			spin_unlock_irqrestore(&engine->sched_engine->lock,
>   					       flags);
>   		}
>   	}
> -	if (rq)
> -		rq = i915_request_get_rcu(rq);
> -
>   	if (!rq)
>   		goto no_request_capture;
>   
>   	capture = intel_engine_coredump_add_request(ee, rq, ATOMIC_MAYFAIL);
> -	if (!capture) {
> -		i915_request_put(rq);
> +	if (!capture)
>   		goto no_request_capture;
> -	}
>   	if (dump_flags & CORE_DUMP_FLAG_IS_GUC_CAPTURE)
>   		intel_guc_capture_get_matching_node(engine->gt, ee, ce);
>   
> @@ -1642,6 +1639,8 @@ capture_engine(struct intel_engine_cs *engine,
>   	return ee;
>   
>   no_request_capture:
> +	if (rq)
> +		i915_request_put(rq);
>   	kfree(ee);
>   	return NULL;
>   }