Re: [PATCH 17/47] drm/i915/guc: Extend deregistration fence to schedule disable

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On 7/9/2021 20:36, Matthew Brost wrote:
On Fri, Jul 09, 2021 at 03:59:11PM -0700, John Harrison wrote:
On 6/24/2021 00:04, Matthew Brost wrote:
Extend the deregistration context fence to fence whne a GuC context has
scheduling disable pending.

Cc: John Harrison <john.c.harrison@xxxxxxxxx>
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
---
   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 37 +++++++++++++++----
   1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
index 0386ccd5a481..0a6ccdf32316 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
@@ -918,7 +918,19 @@ static void guc_context_sched_disable(struct intel_context *ce)
   		goto unpin;
   	spin_lock_irqsave(&ce->guc_state.lock, flags);
+
+	/*
+	 * We have to check if the context has been pinned again as another pin
+	 * operation is allowed to pass this function. Checking the pin count
+	 * here synchronizes this function with guc_request_alloc ensuring a
+	 * request doesn't slip through the 'context_pending_disable' fence.
+	 */
The pin count is an atomic so doesn't need the spinlock. Also the above
How about?

/*
  * We have to check if the context has been pinned again as another pin
  * operation is allowed to pass this function. Checking the pin count,
  * within ce->guc_state.lock, synchronizes this function with
  * guc_request_alloc ensuring a request doesn't slip through the
  * 'context_pending_disable' fence. Checking within the spin lock (can't
  * sleep) ensures another process doesn't pin this context and generate
  * a request before we set the 'context_pending_disable' flag here.
  */

Matt
Sounds good. With that added in:
Reviewed-by: John Harrison <John.C.Harrison@xxxxxxxxx>


comment 'checking the pin count here synchronizes ...' seems wrong. Isn't
the point that acquiring the spinlock is what synchronises with
guc_request_alloc? So the comment should be before the spinlock acquire and
should mention using the spinlock for this purpose?

John.


+	if (unlikely(atomic_add_unless(&ce->pin_count, -2, 2))) {
+		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
+		return;
+	}
   	guc_id = prep_context_pending_disable(ce);
+
   	spin_unlock_irqrestore(&ce->guc_state.lock, flags);
   	with_intel_runtime_pm(runtime_pm, wakeref)
@@ -1123,19 +1135,22 @@ static int guc_request_alloc(struct i915_request *rq)
   out:
   	/*
   	 * We block all requests on this context if a G2H is pending for a
-	 * context deregistration as the GuC will fail a context registration
-	 * while this G2H is pending. Once a G2H returns, the fence is released
-	 * that is blocking these requests (see guc_signal_context_fence).
+	 * schedule disable or context deregistration as the GuC will fail a
+	 * schedule enable or context registration if either G2H is pending
+	 * respectfully. Once a G2H returns, the fence is released that is
+	 * blocking these requests (see guc_signal_context_fence).
   	 *
-	 * We can safely check the below field outside of the lock as it isn't
-	 * possible for this field to transition from being clear to set but
+	 * We can safely check the below fields outside of the lock as it isn't
+	 * possible for these fields to transition from being clear to set but
   	 * converse is possible, hence the need for the check within the lock.
   	 */
-	if (likely(!context_wait_for_deregister_to_register(ce)))
+	if (likely(!context_wait_for_deregister_to_register(ce) &&
+		   !context_pending_disable(ce)))
   		return 0;
   	spin_lock_irqsave(&ce->guc_state.lock, flags);
-	if (context_wait_for_deregister_to_register(ce)) {
+	if (context_wait_for_deregister_to_register(ce) ||
+	    context_pending_disable(ce)) {
   		i915_sw_fence_await(&rq->submit);
   		list_add_tail(&rq->guc_fence_link, &ce->guc_state.fences);
@@ -1484,10 +1499,18 @@ int intel_guc_sched_done_process_msg(struct intel_guc *guc,
   	if (context_pending_enable(ce)) {
   		clr_context_pending_enable(ce);
   	} else if (context_pending_disable(ce)) {
+		/*
+		 * Unpin must be done before __guc_signal_context_fence,
+		 * otherwise a race exists between the requests getting
+		 * submitted + retired before this unpin completes resulting in
+		 * the pin_count going to zero and the context still being
+		 * enabled.
+		 */
   		intel_context_sched_disable_unpin(ce);
   		spin_lock_irqsave(&ce->guc_state.lock, flags);
   		clr_context_pending_disable(ce);
+		__guc_signal_context_fence(ce);
   		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
   	}




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