Re: [Intel-gfx] [PATCH] drm/i915/gt: Close race between enable_breadcrumbs and cancel_breadcrumbs

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

On 19/01/2021 16:20, Chris Wilson wrote:
> If we enable_breadcrumbs for a request while that request is being
> removed from HW; we may see that the request is active as we take the
> ce->signal_lock and proceed to attach the request to ce->signals.
> However, during unsubmission after marking the request as inactive, we
> see that the request has not yet been added to ce->signals and so skip
> the removal. Pull the check during cancel_breadcrumbs under the same
> spinlock as enabling so that we the two tests are consistent in
> enable/cancel.
> 
> Otherwise, we may insert a request onto ce->signal that we expect should
> not be there:
> 
>    intel_context_remove_breadcrumbs:488 GEM_BUG_ON(!__i915_request_is_complete(rq))
> 
> While updating, we can note that we are always called with
> irqs-disabled, due to the engine->active.lock being held at the single
> caller, and so remove the irqsave/restore.
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2931
> Fixes: c18636f76344 ("drm/i915: Remove requirement for holding i915_request.lock for breadcrumbs")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Andi Shyti <andi.shyti@intel.com>
> Cc: <stable@vger.kernel.org> # v5.10+
> ---
>   drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index d098fc0c14ec..34a645d6babd 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -453,16 +453,17 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
>   {
>   	struct intel_breadcrumbs *b = READ_ONCE(rq->engine)->breadcrumbs;
>   	struct intel_context *ce = rq->context;
> -	unsigned long flags;
>   	bool release;
>   
> -	if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
> +	spin_lock(&ce->signal_lock);
> +	if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags)) {
> +		spin_unlock(&ce->signal_lock);
>   		return;
> +	}
>   
> -	spin_lock_irqsave(&ce->signal_lock, flags);
>   	list_del_rcu(&rq->signal_link);
>   	release = remove_signaling_context(b, ce);
> -	spin_unlock_irqrestore(&ce->signal_lock, flags);
> +	spin_unlock(&ce->signal_lock);
>   	if (release)
>   		intel_context_put(ce);
>   
> 

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

Regards,

Tvrtko