drm/i915/gem: Reduce ctx->engines_mutex for get_engines()

Take a snapshot of the ctx->engines, so we can avoid taking the
ctx->engines_mutex for a mere read in get_engines().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114135612.13210-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1843,27 +1843,6 @@ set_engines(struct i915_gem_context *ctx,
 	return 0;
 }
 
-static struct i915_gem_engines *
-__copy_engines(struct i915_gem_engines *e)
-{
-	struct i915_gem_engines *copy;
-	unsigned int n;
-
-	copy = alloc_engines(e->num_engines);
-	if (!copy)
-		return ERR_PTR(-ENOMEM);
-
-	for (n = 0; n < e->num_engines; n++) {
-		if (e->engines[n])
-			copy->engines[n] = intel_context_get(e->engines[n]);
-		else
-			copy->engines[n] = NULL;
-	}
-	copy->num_engines = n;
-
-	return copy;
-}
-
 static int
 get_engines(struct i915_gem_context *ctx,
 	    struct drm_i915_gem_context_param *args)
@@ -1871,19 +1850,17 @@ get_engines(struct i915_gem_context *ctx,
 	struct i915_context_param_engines __user *user;
 	struct i915_gem_engines *e;
 	size_t n, count, size;
+	bool user_engines;
 	int err = 0;
 
-	err = mutex_lock_interruptible(&ctx->engines_mutex);
-	if (err)
-		return err;
+	e = __context_engines_await(ctx, &user_engines);
+	if (!e)
+		return -ENOENT;
 
-	e = NULL;
-	if (i915_gem_context_user_engines(ctx))
-		e = __copy_engines(i915_gem_context_engines(ctx));
-	mutex_unlock(&ctx->engines_mutex);
-	if (IS_ERR_OR_NULL(e)) {
+	if (!user_engines) {
+		i915_sw_fence_complete(&e->fence);
 		args->size = 0;
-		return PTR_ERR_OR_ZERO(e);
+		return 0;
 	}
 
 	count = e->num_engines;
@@ -1934,7 +1911,7 @@ get_engines(struct i915_gem_context *ctx,
 	args->size = size;
 
 err_free:
-	free_engines(e);
+	i915_sw_fence_complete(&e->fence);
 	return err;
 }