Re: [PATCH v3 3/3] drm/i915: Fix premature LRC unpin in GuC mode

[Chris Wilson <chris@chris-wilson.co.uk>]

On Thu, Jan 21, 2016 at 10:10:42AM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> In GuC mode LRC pinning lifetime depends exclusively on the
> request liftime. Since that is terminated by the seqno update
> that opens up a race condition between GPU finishing writing
> out the context image and the driver unpinning the LRC.
> 
> To extend the LRC lifetime we will employ a similar approach
> to what legacy ringbuffer submission does.
> 
> We will start tracking the last submitted context per engine
> and keep it pinned until it is replaced by another one.
> 
> Note that the driver unload path is a bit fragile and could
> benefit greatly from efforts to unify the legacy and exec
> list submission code paths.
> 
> At the moment i915_gem_context_fini has special casing for the
> two which are potentialy not needed, and also depends on
> i915_gem_cleanup_ringbuffer running before itself.
> 
> v2:
>  * Move pinning into engine->emit_request and actually fix
>    the reference/unreference logic. (Chris Wilson)
> 
>  * ring->dev can be NULL on driver unload so use a different
>    route towards it.
> 
> v3:
>  * Rebase.
>  * Handle the reset path. (Chris Wilson)
>  * Exclude default context from the pinning - it is impossible
>    to get it right before default context special casing in
>    general is eliminated.

Since you have the refactoring in place, eliminating the special case is
trivial. So patch 2.5?

Looks like http://patchwork.freedesktop.org/patch/69972/ but you already
have half of that patch in place.

Also if we take a step to add patch 2.1:

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index c25083c78ba7..d43c886fd95a 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -321,6 +321,14 @@ err_destroy:
        return ERR_PTR(ret);
 }
 
+static void i915_gem_context_unpin(struct intel_context *ctx,
+                                  struct intel_engine_cs *rcs)
+{
+       if (rcs->id == RCS && ctx->legacy_hw_ctx.rcs_state)
+               i915_gem_object_ggtt_unpin(ctx->legacy_hw_ctx.rcs_state);
+       i915_gem_context_unreference(ctx);
+}
+
 void i915_gem_context_reset(struct drm_device *dev)
 {
        struct drm_i915_private *dev_priv = dev->dev_private;
@@ -338,13 +346,9 @@ void i915_gem_context_reset(struct drm_device *dev)
 
        for (i = 0; i < I915_NUM_RINGS; i++) {
                struct intel_engine_cs *ring = &dev_priv->ring[i];
-               struct intel_context *lctx = ring->last_context;
 
-               if (lctx) {
-                       if (lctx->legacy_hw_ctx.rcs_state && i == RCS)
-                               i915_gem_object_ggtt_unpin(lctx->legacy_hw_ctx.rcs_state);
-
-                       i915_gem_context_unreference(lctx);
+               if (ring->last_context) {
+                       i915_gem_context_unpin(ring->last_context, ring);
                        ring->last_context = NULL;
                }
 
@@ -424,25 +428,18 @@ void i915_gem_context_fini(struct drm_device *dev)
                 * to offset the do_switch part, so that i915_gem_context_unreference()
                 * can then free the base object correctly. */
                WARN_ON(!dev_priv->ring[RCS].last_context);
-               if (dev_priv->ring[RCS].last_context == dctx) {
-                       /* Fake switch to NULL context */
-                       WARN_ON(dctx->legacy_hw_ctx.rcs_state->active);
-                       i915_gem_object_ggtt_unpin(dctx->legacy_hw_ctx.rcs_state);
-                       i915_gem_context_unreference(dctx);
-                       dev_priv->ring[RCS].last_context = NULL;
-               }
-
                i915_gem_object_ggtt_unpin(dctx->legacy_hw_ctx.rcs_state);
        }
 
        for (i = 0; i < I915_NUM_RINGS; i++) {
                struct intel_engine_cs *ring = &dev_priv->ring[i];
 
-               if (ring->last_context)
-                       i915_gem_context_unreference(ring->last_context);
+               if (ring->last_context) {
+                       i915_gem_context_unpin(ring->last_context, ring);
+                       ring->last_context = NULL;
+               }
 
                ring->default_context = NULL;
-               ring->last_context = NULL;
        }
 
        i915_gem_context_unreference(dctx);

Then we just end up with

if (i915.enable_execlists)
	intel_lr_context_unpin(ring->last_context, ring);
else
	i915_gem_context_unpin(ring->last_context, ring);

which is much less messy in this patch and one step closer to unifying
engine->pin_context()/engine->unpin_context() vfuns.

The only tricky part there is having to disentangle the legacy paths,
but doing so should allow us to see clearly that the sequence is just

intel_gpu_reset();
for_each_engine() unpin(engine->last_context);
unpin(dev_priv->default_context);

After a few more passes, we should be able to call the reset and cleanup
functions higher up (as part of the stopping the GPU upon suspend
procedure before tearing down the sw structs) at which point we only
need the unpin(default_context) here.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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