From 9ddc8ec027a39759c946c3f4944d3e0c5a007ccd Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Mon, 16 Dec 2019 16:53:17 +0000
Subject: [PATCH] drm/i915: Eliminate the trylock for awaiting an earlier request
Git-commit: 9ddc8ec027a39759c946c3f4944d3e0c5a007ccd
Patch-mainline: v5.6-rc1
References: bsc#1174737
We currently use an error-prone mutex_trylock to grab another timeline
to find an earlier request along it. However, with a bit of a
sleight-of-hand, we can reduce the mutex_trylock to a spin_lock on the
immediate request and careful pointer chasing to acquire a reference on
the previous request.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216165317.2742896-1-chris@chris-wilson.co.uk
Acked-by: Takashi Iwai <tiwai@suse.de>
---
drivers/gpu/drm/i915/i915_request.c | 39 +++++++++++++++++++-----------------
1 file changed, 21 insertions(+), 18 deletions(-)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -785,34 +785,37 @@ err_unlock:
static int
i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
{
- struct intel_timeline *tl;
struct dma_fence *fence;
int err;
GEM_BUG_ON(i915_request_timeline(rq) ==
rcu_access_pointer(signal->timeline));
+ fence = NULL;
rcu_read_lock();
- tl = rcu_dereference(signal->timeline);
- if (i915_request_started(signal) || !kref_get_unless_zero(&tl->kref))
- tl = NULL;
- rcu_read_unlock();
- if (!tl) /* already started or maybe even completed */
- return 0;
+ spin_lock_irq(&signal->lock);
+ if (!i915_request_started(signal) &&
+ !list_is_first(&signal->link,
+ &rcu_dereference(signal->timeline)->requests)) {
+ struct i915_request *prev = list_prev_entry(signal, link);
- fence = ERR_PTR(-EAGAIN);
- if (mutex_trylock(&tl->mutex)) {
- fence = NULL;
- if (!i915_request_started(signal) &&
- !list_is_first(&signal->link, &tl->requests)) {
- signal = list_prev_entry(signal, link);
- fence = dma_fence_get(&signal->fence);
+ /*
+ * Peek at the request before us in the timeline. That
+ * request will only be valid before it is retired, so
+ * after acquiring a reference to it, confirm that it is
+ * still part of the signaler's timeline.
+ */
+ if (i915_request_get_rcu(prev)) {
+ if (list_next_entry(prev, link) == signal)
+ fence = &prev->fence;
+ else
+ i915_request_put(prev);
}
- mutex_unlock(&tl->mutex);
}
- intel_timeline_put(tl);
- if (IS_ERR_OR_NULL(fence))
- return PTR_ERR_OR_ZERO(fence);
+ spin_unlock_irq(&signal->lock);
+ rcu_read_unlock();
+ if (!fence)
+ return 0;
err = 0;
if (intel_timeline_sync_is_later(i915_request_timeline(rq), fence))