On Tue, Jan 02, 2018 at 03:12:26PM +0000, Chris Wilson wrote:
> The dependency chain must be an acyclic graph. This is checked by the
> swfence, but for sanity, also do a simple check that we do not corrupt
> our list iteration in execlists_schedule() by a shallow dependency
> cycle.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Michał Winiarski <michal.winiarski@xxxxxxxxx>
-Michał
> ---
> drivers/gpu/drm/i915/intel_lrc.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 007aec9d95c9..8c9d6cef2482 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -1006,7 +1006,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
> stack.signaler = &request->priotree;
> list_add(&stack.dfs_link, &dfs);
>
> - /* Recursively bump all dependent priorities to match the new request.
> + /*
> + * Recursively bump all dependent priorities to match the new request.
> *
> * A naive approach would be to use recursion:
> * static void update_priorities(struct i915_priotree *pt, prio) {
> @@ -1026,12 +1027,15 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
> list_for_each_entry_safe(dep, p, &dfs, dfs_link) {
> struct i915_priotree *pt = dep->signaler;
>
> - /* Within an engine, there can be no cycle, but we may
> + /*
> + * Within an engine, there can be no cycle, but we may
> * refer to the same dependency chain multiple times
> * (redundant dependencies are not eliminated) and across
> * engines.
> */
> list_for_each_entry(p, &pt->signalers_list, signal_link) {
> + GEM_BUG_ON(p == dep); /* no cycles! */
> +
> if (i915_gem_request_completed(priotree_to_request(p->signaler)))
> continue;
>
> @@ -1043,7 +1047,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
> list_safe_reset_next(dep, p, dfs_link);
> }
>
> - /* If we didn't need to bump any existing priorities, and we haven't
> + /*
> + * If we didn't need to bump any existing priorities, and we haven't
> * yet submitted this request (i.e. there is no potential race with
> * execlists_submit_request()), we can set our own priority and skip
> * acquiring the engine locks.
> --
> 2.15.1
>
> _______________________________________________
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