Re: [PATCH] drm/i915: Lock out execlist tasklet while peeking inside for busy-stats

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 15/01/2018 09:20, Chris Wilson wrote:
> In order to prevent a race condition where we may end up overaccounting
> the active state and leaving the busy-stats believing the GPU is 100%
> busy, lock out the tasklet while we reconstruct the busy state. There is
> no direct spinlock guard for the execlists->port[], so we need to
> utilise tasklet_disable() as a synchronous barrier to prevent the only
> writer to execlists->port[] from running at the same time as the enable.
> 
> Fixes: 4900727d35bb ("drm/i915/pmu: Reconstruct active state on starting busy-stats")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/intel_engine_cs.c | 20 ++++++++++++--------
>   1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
> index d790bdc227ff..b221610f2365 100644
> --- a/drivers/gpu/drm/i915/intel_engine_cs.c
> +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
> @@ -1943,16 +1943,22 @@ intel_engine_lookup_user(struct drm_i915_private *i915, u8 class, u8 instance)
>    */
>   int intel_enable_engine_stats(struct intel_engine_cs *engine)
>   {
> +	struct intel_engine_execlists *execlists = &engine->execlists;
>   	unsigned long flags;
> +	int err = 0;
>   
>   	if (!intel_engine_supports_stats(engine))
>   		return -ENODEV;
>   
> +	tasklet_disable(&execlists->tasklet);
>   	spin_lock_irqsave(&engine->stats.lock, flags);
> -	if (engine->stats.enabled == ~0)
> -		goto busy;
> +
> +	if (unlikely(engine->stats.enabled == ~0)) {
> +		err = -EBUSY;
> +		goto unlock;
> +	}
> +
>   	if (engine->stats.enabled++ == 0) {
> -		struct intel_engine_execlists *execlists = &engine->execlists;
>   		const struct execlist_port *port = execlists->port;
>   		unsigned int num_ports = execlists_num_ports(execlists);
>   
> @@ -1967,14 +1973,12 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine)
>   		if (engine->stats.active)
>   			engine->stats.start = engine->stats.enabled_at;
>   	}
> -	spin_unlock_irqrestore(&engine->stats.lock, flags);
>   
> -	return 0;
> -
> -busy:
> +unlock:
>   	spin_unlock_irqrestore(&engine->stats.lock, flags);
> +	tasklet_enable(&execlists->tasklet);
>   
> -	return -EBUSY;
> +	return err;
>   }
>   
>   static ktime_t __intel_engine_get_busy_time(struct intel_engine_cs *engine)
> 

Hopefully the end of the immediate trouble, with only beautification 
remaining.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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