[RFC] PM / QoS: Decouple request alloc from dev_pm_qos_mtx (alternative solution)

[Rob Clark <robdclark@gmail.com>]

From: Rob Clark <robdclark@chromium.org>

Similar to the previous patch, move the allocation out from under
dev_pm_qos_mtx, by speculatively doing the allocation and handle
any race after acquiring dev_pm_qos_mtx by freeing the redundant
allocation.

Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
---
This is an alternative to https://patchwork.freedesktop.org/patch/551417/?series=115028&rev=4

So, this does _slightly_ change error paths, for ex
dev_pm_qos_update_user_latency_tolerance() will now allocate
dev->power.qos in some error cases.  But this seems harmless?
A slightly more complicated version of this could conserve the
previous error path behavior, but I figured I'd try the simpler
thing first.

 drivers/base/power/qos.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/base/power/qos.c b/drivers/base/power/qos.c
index 1b73a704aac1..c7ba85e89c42 100644
--- a/drivers/base/power/qos.c
+++ b/drivers/base/power/qos.c
@@ -920,8 +920,12 @@ s32 dev_pm_qos_get_user_latency_tolerance(struct device *dev)
 int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
 {
 	struct dev_pm_qos *qos = dev_pm_qos_constraints_allocate();
+	struct dev_pm_qos_request *req = NULL;
 	int ret = 0;
 
+	if (!dev->power.qos->latency_tolerance_req)
+		req = kzalloc(sizeof(*req), GFP_KERNEL);
+
 	mutex_lock(&dev_pm_qos_mtx);
 
 	dev_pm_qos_constraints_set(dev, qos);
@@ -935,8 +939,6 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
 		goto out;
 
 	if (!dev->power.qos->latency_tolerance_req) {
-		struct dev_pm_qos_request *req;
-
 		if (val < 0) {
 			if (val == PM_QOS_LATENCY_TOLERANCE_NO_CONSTRAINT)
 				ret = 0;
@@ -944,17 +946,15 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
 				ret = -EINVAL;
 			goto out;
 		}
-		req = kzalloc(sizeof(*req), GFP_KERNEL);
 		if (!req) {
 			ret = -ENOMEM;
 			goto out;
 		}
 		ret = __dev_pm_qos_add_request(dev, req, DEV_PM_QOS_LATENCY_TOLERANCE, val);
-		if (ret < 0) {
-			kfree(req);
+		if (ret < 0)
 			goto out;
-		}
 		dev->power.qos->latency_tolerance_req = req;
+		req = NULL;
 	} else {
 		if (val < 0) {
 			__dev_pm_qos_drop_user_request(dev, DEV_PM_QOS_LATENCY_TOLERANCE);
@@ -966,6 +966,7 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
 
  out:
 	mutex_unlock(&dev_pm_qos_mtx);
+	kfree(req);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(dev_pm_qos_update_user_latency_tolerance);
-- 
2.41.0