Re: [PATCH 4/6] drm/i915/breadcrumbs: Disable interrupt bottom-half first on idling

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

On 15/03/2017 14:01, Chris Wilson wrote:
> Before walking the rbtree of waiters (marking them as complete and waking
> them), decouple the interrupt handler. This prevents a race between the
> missed waiter waking up and removing its intel_wait (which skips
> checking the lock) and the interrupt handler dereferencing the
> intel_wait. (Though we do not expect to encounter waiters during idle!)
>
> Fixes: e1c0c91bdaec ("drm/i915: Wake up all waiters before idling")
> 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/intel_breadcrumbs.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> index 31e7c25013a4..ebcb595001fc 100644
> --- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> @@ -179,7 +179,7 @@ void __intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
>  void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
>  {
>  	struct intel_breadcrumbs *b = &engine->breadcrumbs;
> -	struct intel_wait *wait, *n;
> +	struct intel_wait *wait, *n, *first;
>
>  	if (!b->irq_armed)
>  		return;
> @@ -190,18 +190,19 @@ void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
>  	 */
>
>  	spin_lock_irq(&b->rb_lock);
> +
> +	spin_lock(&b->irq_lock);
> +	first = fetch_and_zero(&b->irq_wait);
> +	__intel_engine_disarm_breadcrumbs(engine);
> +	spin_unlock(&b->irq_lock);
> +
>  	rbtree_postorder_for_each_entry_safe(wait, n, &b->waiters, node) {
>  		RB_CLEAR_NODE(&wait->node);
> -		if (wake_up_process(wait->tsk) && wait == b->irq_wait)
> +		if (wake_up_process(wait->tsk) && wait == first)
>  			missed_breadcrumb(engine);
>  	}
>  	b->waiters = RB_ROOT;
>
> -	spin_lock(&b->irq_lock);
> -	b->irq_wait = NULL;
> -	__intel_engine_disarm_breadcrumbs(engine);
> -	spin_unlock(&b->irq_lock);
> -
>  	spin_unlock_irq(&b->rb_lock);
>  }
>
>

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

Regards,

Tvrtko
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