Re: [PATCH] drm/i915: Park the breadcrumbs signaler across a GPU reset

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> The signal threads may be running concurrently with the GPU reset. The
> completion from the GPU run asynchronous with the reset and two threads
> may see different snapshots of the state, and the signaler may mark a
> request as complete as we try to reset it. We don't tolerate 2 different
> views of the same state and complain if we try to mark a request as
> failed if it is already complete. Disable the signal threads during
> reset to prevent this conflict (even though the conflict implies that
> the state we resetting to is invalid, we have already made our
> decision!).
>
> References: https://bugs.freedesktop.org/show_bug.cgi?id=99671
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c          | 16 +++++++++++++++-
>  drivers/gpu/drm/i915/intel_breadcrumbs.c |  3 +++
>  2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 48922ff454e6..95582295b219 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -35,6 +35,7 @@
>  #include "intel_frontbuffer.h"
>  #include "intel_mocs.h"
>  #include <linux/dma-fence-array.h>
> +#include <linux/kthread.h>
>  #include <linux/reservation.h>
>  #include <linux/shmem_fs.h>
>  #include <linux/slab.h>
> @@ -2643,6 +2644,17 @@ int i915_gem_reset_prepare(struct drm_i915_private *dev_priv)
>  	for_each_engine(engine, dev_priv, id) {
>  		struct drm_i915_gem_request *request;
>  
> +		/* Prevent the signaler thread from updating the request
> +		 * state (by calling dma_fence_signal) as we are processing
> +		 * the reset. The write from the GPU of the seqno is
> +		 * asynchronous and the signaler thread may see a different
> +		 * value to us and declare the request complete, even though
> +		 * the reset routine have picked that request as the active
> +		 * (incomplete) request. This conflict is not handled
> +		 * gracefully!
> +		 */
> +		kthread_park(engine->breadcrumbs.signaler);
> +

I was wondering here if it would be best to disable the tasklet first.
But stopping the car before we stop the engine seems wise thing to do.

Another unrelated thing is that we might be better off just to
make find_active_request always seqno coherent and fork a
noncoherent version for error capture.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

>  		/* Prevent request submission to the hardware until we have
>  		 * completed the reset in i915_gem_reset_finish(). If a request
>  		 * is completed by one engine, it may then queue a request
> @@ -2796,8 +2808,10 @@ void i915_gem_reset_finish(struct drm_i915_private *dev_priv)
>  
>  	lockdep_assert_held(&dev_priv->drm.struct_mutex);
>  
> -	for_each_engine(engine, dev_priv, id)
> +	for_each_engine(engine, dev_priv, id) {
>  		tasklet_enable(&engine->irq_tasklet);
> +		kthread_unpark(engine->breadcrumbs.signaler);
> +	}
>  }
>  
>  static void nop_submit_request(struct drm_i915_gem_request *request)
> diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> index 9fd002bcebb6..f5e05343110a 100644
> --- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> @@ -490,6 +490,9 @@ static int intel_breadcrumbs_signaler(void *arg)
>  				break;
>  
>  			schedule();
> +
> +			if (kthread_should_park())
> +				kthread_parkme();
>  		}
>  	} while (1);
>  	__set_current_state(TASK_RUNNING);
> -- 
> 2.11.0
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