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 8/24/2021 6:44 PM, Matthew Brost wrote:
> On Tue, Aug 24, 2021 at 06:20:49PM -0700, Daniele Ceraolo Spurio wrote:
>>
>> 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.
>>
> Right, hence the comment in the code.
>   
>>> +	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?
>>
> Commmit message cover this - this is dead code that can't be reached
> in the current code nor can be it be reached in upcoming code. Or put
> another way loop is always true thus we can't get -EBUSY from
> deregister_context().

ok, I hadn't realized that we could get -EBUSY only if loop=false.

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>

Daniele

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