drm/i915: Nuke struct_mutex from context_setparam

Userspace should be free to race against itself and shoot itself in
the foot if it so desires to adjust a parameter at the same time as
submitting a batch to that context. As such, the struct_mutex in context
setparam is only being used to serialise userspace against itself and
not for any protection of internal structs and so is superfluous.

v2: Separate user_flags from internal flags to reduce chance of
interference; and use locked bit ops for user updates.

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/20180911132206.23032-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_context.c

@@ -862,7 +862,7 @@ int i915_gem_context_getparam_ioctl(struct drm_device *dev, void *data,
 		ret = -EINVAL;
 		break;
 	case I915_CONTEXT_PARAM_NO_ZEROMAP:
-		args->value = ctx->flags & CONTEXT_NO_ZEROMAP;
+		args->value = test_bit(UCONTEXT_NO_ZEROMAP, &ctx->user_flags);
 		break;
 	case I915_CONTEXT_PARAM_GTT_SIZE:
 		if (ctx->ppgtt)
@@ -896,27 +896,23 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 	struct drm_i915_file_private *file_priv = file->driver_priv;
 	struct drm_i915_gem_context_param *args = data;
 	struct i915_gem_context *ctx;
-	int ret;
+	int ret = 0;
 
 	ctx = i915_gem_context_lookup(file_priv, args->ctx_id);
 	if (!ctx)
 		return -ENOENT;
 
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		goto out;
-
 	switch (args->param) {
 	case I915_CONTEXT_PARAM_BAN_PERIOD:
 		ret = -EINVAL;
 		break;
 	case I915_CONTEXT_PARAM_NO_ZEROMAP:
-		if (args->size) {
+		if (args->size)
 			ret = -EINVAL;
-		} else {
-			ctx->flags &= ~CONTEXT_NO_ZEROMAP;
-			ctx->flags |= args->value ? CONTEXT_NO_ZEROMAP : 0;
-		}
+		else if (args->value)
+			set_bit(UCONTEXT_NO_ZEROMAP, &ctx->user_flags);
+		else
+			clear_bit(UCONTEXT_NO_ZEROMAP, &ctx->user_flags);
 		break;
 	case I915_CONTEXT_PARAM_NO_ERROR_CAPTURE:
 		if (args->size)
@@ -960,9 +956,7 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 		ret = -EINVAL;
 		break;
 	}
-	mutex_unlock(&dev->struct_mutex);
 
-out:
 	i915_gem_context_put(ctx);
 	return ret;
 }

drivers/gpu/drm/i915/i915_gem_context.h

@@ -116,16 +116,21 @@ struct i915_gem_context {
 	 */
 	struct rcu_head rcu;
 
+	/**
+	 * @user_flags: small set of booleans controlled by the user
+	 */
+	unsigned long user_flags;
+#define UCONTEXT_NO_ZEROMAP		0
+#define UCONTEXT_NO_ERROR_CAPTURE	1
+#define UCONTEXT_BANNABLE		2
+
 	/**
 	 * @flags: small set of booleans
 	 */
 	unsigned long flags;
-#define CONTEXT_NO_ZEROMAP		BIT(0)
-#define CONTEXT_NO_ERROR_CAPTURE	1
-#define CONTEXT_CLOSED			2
-#define CONTEXT_BANNABLE		3
-#define CONTEXT_BANNED			4
-#define CONTEXT_FORCE_SINGLE_SUBMISSION	5
+#define CONTEXT_BANNED			0
+#define CONTEXT_CLOSED			1
+#define CONTEXT_FORCE_SINGLE_SUBMISSION	2
 
 	/**
 	 * @hw_id: - unique identifier for the context
@@ -209,37 +214,37 @@ static inline bool i915_gem_context_is_closed(const struct i915_gem_context *ctx
 static inline void i915_gem_context_set_closed(struct i915_gem_context *ctx)
 {
 	GEM_BUG_ON(i915_gem_context_is_closed(ctx));
-	__set_bit(CONTEXT_CLOSED, &ctx->flags);
+	set_bit(CONTEXT_CLOSED, &ctx->flags);
 }
 
 static inline bool i915_gem_context_no_error_capture(const struct i915_gem_context *ctx)
 {
-	return test_bit(CONTEXT_NO_ERROR_CAPTURE, &ctx->flags);
+	return test_bit(UCONTEXT_NO_ERROR_CAPTURE, &ctx->user_flags);
 }
 
 static inline void i915_gem_context_set_no_error_capture(struct i915_gem_context *ctx)
 {
-	__set_bit(CONTEXT_NO_ERROR_CAPTURE, &ctx->flags);
+	set_bit(UCONTEXT_NO_ERROR_CAPTURE, &ctx->user_flags);
 }
 
 static inline void i915_gem_context_clear_no_error_capture(struct i915_gem_context *ctx)
 {
-	__clear_bit(CONTEXT_NO_ERROR_CAPTURE, &ctx->flags);
+	clear_bit(UCONTEXT_NO_ERROR_CAPTURE, &ctx->user_flags);
 }
 
 static inline bool i915_gem_context_is_bannable(const struct i915_gem_context *ctx)
 {
-	return test_bit(CONTEXT_BANNABLE, &ctx->flags);
+	return test_bit(UCONTEXT_BANNABLE, &ctx->user_flags);
 }
 
 static inline void i915_gem_context_set_bannable(struct i915_gem_context *ctx)
 {
-	__set_bit(CONTEXT_BANNABLE, &ctx->flags);
+	set_bit(UCONTEXT_BANNABLE, &ctx->user_flags);
 }
 
 static inline void i915_gem_context_clear_bannable(struct i915_gem_context *ctx)
 {
-	__clear_bit(CONTEXT_BANNABLE, &ctx->flags);
+	clear_bit(UCONTEXT_BANNABLE, &ctx->user_flags);
 }
 
 static inline bool i915_gem_context_is_banned(const struct i915_gem_context *ctx)
@@ -249,7 +254,7 @@ static inline bool i915_gem_context_is_banned(const struct i915_gem_context *ctx
 
 static inline void i915_gem_context_set_banned(struct i915_gem_context *ctx)
 {
-	__set_bit(CONTEXT_BANNED, &ctx->flags);
+	set_bit(CONTEXT_BANNED, &ctx->flags);
 }
 
 static inline bool i915_gem_context_force_single_submission(const struct i915_gem_context *ctx)

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -743,7 +743,7 @@ static int eb_select_context(struct i915_execbuffer *eb)
 	}
 
 	eb->context_flags = 0;
-	if (ctx->flags & CONTEXT_NO_ZEROMAP)
+	if (test_bit(UCONTEXT_NO_ZEROMAP, &ctx->user_flags))
 		eb->context_flags |= __EXEC_OBJECT_NEEDS_BIAS;
 
 	return 0;