[PATCH 1/1] HID: hidraw: Replace hidraw device table mutex with a rwsem

[André Almeida]

Currently, the table that stores information about the connected hidraw
devices has a mutex to prevent concurrent hidraw users to manipulate the
hidraw table (e.g. delete an entry) while someone is trying to use
the table (e.g. issuing an ioctl to the device), preventing the kernel
to referencing a NULL pointer. However, since that every user that wants
to access the table for both manipulating it and reading it content,
this prevents concurrent access to the table for read-only operations
for different or the same device (e.g. two hidraw ioctls can't happen at
the same time, even if they are completely unrelated).

This proves to be a bottleneck and gives performance issues when using
multiple HID devices at same time, like VR kits where one can have two
controllers, the headset and some tracking sensors.

To improve the performance, replace the table mutex with a read-write
semaphore, enabling multiple threads to issue parallel syscalls to
multiple devices at the same time while protecting the table for
concurrent modifications.

Signed-off-by: André Almeida <andrealmeid@xxxxxxxxxxxxx>
---
 drivers/hid/hidraw.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/drivers/hid/hidraw.c b/drivers/hid/hidraw.c
index 79faac87a06f..681614a8302a 100644
--- a/drivers/hid/hidraw.c
+++ b/drivers/hid/hidraw.c
@@ -34,7 +34,7 @@ static int hidraw_major;
 static struct cdev hidraw_cdev;
 static struct class *hidraw_class;
 static struct hidraw *hidraw_table[HIDRAW_MAX_DEVICES];
-static DEFINE_MUTEX(minors_lock);
+static DECLARE_RWSEM(minors_rwsem);
 
 static ssize_t hidraw_read(struct file *file, char __user *buffer, size_t count, loff_t *ppos)
 {
@@ -107,7 +107,7 @@ static ssize_t hidraw_send_report(struct file *file, const char __user *buffer,
 	__u8 *buf;
 	int ret = 0;
 
-	lockdep_assert_held(&minors_lock);
+	lockdep_assert_held(&minors_rwsem);
 
 	if (!hidraw_table[minor] || !hidraw_table[minor]->exist) {
 		ret = -ENODEV;
@@ -160,9 +160,9 @@ static ssize_t hidraw_send_report(struct file *file, const char __user *buffer,
 static ssize_t hidraw_write(struct file *file, const char __user *buffer, size_t count, loff_t *ppos)
 {
 	ssize_t ret;
-	mutex_lock(&minors_lock);
+	down_read(&minors_rwsem);
 	ret = hidraw_send_report(file, buffer, count, HID_OUTPUT_REPORT);
-	mutex_unlock(&minors_lock);
+	up_read(&minors_rwsem);
 	return ret;
 }
 
@@ -182,7 +182,7 @@ static ssize_t hidraw_get_report(struct file *file, char __user *buffer, size_t
 	int ret = 0, len;
 	unsigned char report_number;
 
-	lockdep_assert_held(&minors_lock);
+	lockdep_assert_held(&minors_rwsem);
 
 	if (!hidraw_table[minor] || !hidraw_table[minor]->exist) {
 		ret = -ENODEV;
@@ -272,7 +272,7 @@ static int hidraw_open(struct inode *inode, struct file *file)
 		goto out;
 	}
 
-	mutex_lock(&minors_lock);
+	down_read(&minors_rwsem);
 	if (!hidraw_table[minor] || !hidraw_table[minor]->exist) {
 		err = -ENODEV;
 		goto out_unlock;
@@ -301,7 +301,7 @@ static int hidraw_open(struct inode *inode, struct file *file)
 	spin_unlock_irqrestore(&hidraw_table[minor]->list_lock, flags);
 	file->private_data = list;
 out_unlock:
-	mutex_unlock(&minors_lock);
+	up_read(&minors_rwsem);
 out:
 	if (err < 0)
 		kfree(list);
@@ -347,7 +347,7 @@ static int hidraw_release(struct inode * inode, struct file * file)
 	struct hidraw_list *list = file->private_data;
 	unsigned long flags;
 
-	mutex_lock(&minors_lock);
+	down_write(&minors_rwsem);
 
 	spin_lock_irqsave(&hidraw_table[minor]->list_lock, flags);
 	list_del(&list->node);
@@ -356,7 +356,7 @@ static int hidraw_release(struct inode * inode, struct file * file)
 
 	drop_ref(hidraw_table[minor], 0);
 
-	mutex_unlock(&minors_lock);
+	up_write(&minors_rwsem);
 	return 0;
 }
 
@@ -369,7 +369,7 @@ static long hidraw_ioctl(struct file *file, unsigned int cmd,
 	struct hidraw *dev;
 	void __user *user_arg = (void __user*) arg;
 
-	mutex_lock(&minors_lock);
+	down_read(&minors_rwsem);
 	dev = hidraw_table[minor];
 	if (!dev || !dev->exist) {
 		ret = -ENODEV;
@@ -487,7 +487,7 @@ static long hidraw_ioctl(struct file *file, unsigned int cmd,
 		ret = -ENOTTY;
 	}
 out:
-	mutex_unlock(&minors_lock);
+	up_read(&minors_rwsem);
 	return ret;
 }
 
@@ -546,7 +546,7 @@ int hidraw_connect(struct hid_device *hid)
 
 	result = -EINVAL;
 
-	mutex_lock(&minors_lock);
+	down_write(&minors_rwsem);
 
 	for (minor = 0; minor < HIDRAW_MAX_DEVICES; minor++) {
 		if (hidraw_table[minor])
@@ -557,7 +557,7 @@ int hidraw_connect(struct hid_device *hid)
 	}
 
 	if (result) {
-		mutex_unlock(&minors_lock);
+		up_write(&minors_rwsem);
 		kfree(dev);
 		goto out;
 	}
@@ -567,7 +567,7 @@ int hidraw_connect(struct hid_device *hid)
 
 	if (IS_ERR(dev->dev)) {
 		hidraw_table[minor] = NULL;
-		mutex_unlock(&minors_lock);
+		up_write(&minors_rwsem);
 		result = PTR_ERR(dev->dev);
 		kfree(dev);
 		goto out;
@@ -583,7 +583,7 @@ int hidraw_connect(struct hid_device *hid)
 	dev->exist = 1;
 	hid->hidraw = dev;
 
-	mutex_unlock(&minors_lock);
+	up_write(&minors_rwsem);
 out:
 	return result;
 
@@ -594,11 +594,11 @@ void hidraw_disconnect(struct hid_device *hid)
 {
 	struct hidraw *hidraw = hid->hidraw;
 
-	mutex_lock(&minors_lock);
+	down_write(&minors_rwsem);
 
 	drop_ref(hidraw, 1);
 
-	mutex_unlock(&minors_lock);
+	up_write(&minors_rwsem);
 }
 EXPORT_SYMBOL_GPL(hidraw_disconnect);
 
-- 
2.34.1