From 3d92c251c04b1b4c6363018220af42ec3a294d1e Mon Sep 17 00:00:00 2001
From: Kenny Levinsen <kl@kl.wtf>
Date: Sun, 27 Dec 2020 14:43:13 +0000
Subject: ZEN: Input: evdev - use call_rcu when detaching client

Significant time was spent on synchronize_rcu in evdev_detach_client
when applications closed evdev devices. Switching VT away from a
graphical environment commonly leads to mass input device closures,
which could lead to noticable delays on systems with many input devices.

Replace synchronize_rcu with call_rcu, deferring reclaim of the evdev
client struct till after the RCU grace period instead of blocking the
calling application.

While this does not solve all slow evdev fd closures, it takes care of a
good portion of them, including this simple test:

	#include <fcntl.h>
	#include <unistd.h>

	int main(int argc, char *argv[])
	{
		int idx, fd;
		const char *path = "/dev/input/event0";
		for (idx = 0; idx < 1000; idx++) {
			if ((fd = open(path, O_RDWR)) == -1) {
				return -1;
			}
			close(fd);
		}
		return 0;
	}

Time to completion of above test when run locally:

	Before: 0m27.111s
	After:  0m0.018s

Signed-off-by: Kenny Levinsen <kl@kl.wtf>
---
 drivers/input/evdev.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

--- a/drivers/input/evdev.c
+++ b/drivers/input/evdev.c
@@ -46,6 +46,7 @@ struct evdev_client {
 	struct fasync_struct *fasync;
 	struct evdev *evdev;
 	struct list_head node;
+	struct rcu_head rcu;
 	enum input_clock_type clk_type;
 	bool revoked;
 	unsigned long *evmasks[EV_CNT];
@@ -368,13 +369,22 @@ static void evdev_attach_client(struct e
 	spin_unlock(&evdev->client_lock);
 }
 
+static void evdev_reclaim_client(struct rcu_head *rp)
+{
+	struct evdev_client *client = container_of(rp, struct evdev_client, rcu);
+	unsigned int i;
+	for (i = 0; i < EV_CNT; ++i)
+		bitmap_free(client->evmasks[i]);
+	kvfree(client);
+}
+
 static void evdev_detach_client(struct evdev *evdev,
 				struct evdev_client *client)
 {
 	spin_lock(&evdev->client_lock);
 	list_del_rcu(&client->node);
 	spin_unlock(&evdev->client_lock);
-	synchronize_rcu();
+	call_rcu(&client->rcu, evdev_reclaim_client);
 }
 
 static int evdev_open_device(struct evdev *evdev)
@@ -427,7 +437,6 @@ static int evdev_release(struct inode *i
 {
 	struct evdev_client *client = file->private_data;
 	struct evdev *evdev = client->evdev;
-	unsigned int i;
 
 	mutex_lock(&evdev->mutex);
 
@@ -439,11 +448,6 @@ static int evdev_release(struct inode *i
 
 	evdev_detach_client(evdev, client);
 
-	for (i = 0; i < EV_CNT; ++i)
-		bitmap_free(client->evmasks[i]);
-
-	kvfree(client);
-
 	evdev_close_device(evdev);
 
 	return 0;
@@ -486,7 +490,6 @@ static int evdev_open(struct inode *inod
 
  err_free_client:
 	evdev_detach_client(evdev, client);
-	kvfree(client);
 	return error;
 }