[Intel-gfx] [PATCH] drm/i915/execlists: Use a local lock for dfs_link access

[Tvrtko Ursulin]

On 16/11/2016 15:27, Chris Wilson wrote:
> Avoid requiring struct_mutex for exclusive access to the temporary
> dfs_link inside the i915_dependency as not all callers may want to touch
> struct_mutex. So rather than force them to take a highly contended
> lock, introduce a local lock for the execlists schedule operation.
>
> Reported-by: David Weinehall <david.weinehall at linux.intel.com>
> Fixes: 9a151987d709 ("drm/i915: Add execution priority boosting for mmioflips")

Grumble grumble, sloppy review. :I

> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: David Weinehall <david.weinehall at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index e23b6a2600fb..10e59ff0d8f1 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -694,6 +694,7 @@ pt_lock_engine(struct i915_priotree *pt, struct intel_engine_cs *locked)
>
>  static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>  {
> +	static DEFINE_MUTEX(lock);

Good enough for one GPU. :) Consider improving in the future as it is 
not in the spirit of the driver.

>  	struct intel_engine_cs *engine = NULL;
>  	struct i915_dependency *dep, *p;
>  	struct i915_dependency stack;
> @@ -702,8 +703,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>  	if (prio <= READ_ONCE(request->priotree.priority))
>  		return;
>
> -	/* Need BKL in order to use the temporary link inside i915_dependency */
> -	lockdep_assert_held(&request->i915->drm.struct_mutex);
> +	/* Need global lock to use the temporary link inside i915_dependency */
> +	mutex_lock(&lock);
>
>  	stack.signaler = &request->priotree;
>  	list_add(&stack.dfs_link, &dfs);
> @@ -770,6 +771,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>  	if (engine)
>  		spin_unlock_irq(&engine->timeline->lock);
>
> +	mutex_unlock(&lock);
> +
>  	/* XXX Do we need to preempt to make room for us and our deps? */
>  }
>
>

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

Regards,

Tvrtko