RFC virtio-rng: fail to read sysfs of a busy device

[Amos Kong <akong@redhat.com>]

(Resend to fix the subject)

Hi Amit, Rusty

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1127062
steps:
- Read random data by 'dd if=/dev/hwrng of=/dev/null' in guest
- check sysfs files in the same time, 'cat /sys/class/misc/hw_random/rng_*'

Result: cat process will get stuck, it will return if we kill dd process.

We have some static variables (eg, current_rng, data_avail, etc) in hw_random/core.c,
they are protected by rng_mutex. I try to workaround this issue by undelay(100)
after mutex_unlock() in rng_dev_read(). This gives chance for hwrng_attr_*_show()
to get mutex.

This patch also contains some cleanup, moving some code out of mutex
protection.

Do you have some suggestion? Thanks.


diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index aa30a25..fa69020 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -194,6 +194,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 		}
 
 		mutex_unlock(&rng_mutex);
+		udelay(100);
 
 		if (need_resched())
 			schedule_timeout_interruptible(1);
@@ -233,10 +234,10 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
 	int err;
 	struct hwrng *rng;
 
+	err = -ENODEV;
 	err = mutex_lock_interruptible(&rng_mutex);
 	if (err)
 		return -ERESTARTSYS;
-	err = -ENODEV;
 	list_for_each_entry(rng, &rng_list, list) {
 		if (strcmp(rng->name, buf) == 0) {
 			if (rng == current_rng) {
@@ -270,8 +271,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
 		return -ERESTARTSYS;
 	if (current_rng)
 		name = current_rng->name;
-	ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
 	mutex_unlock(&rng_mutex);
+	ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
 
 	return ret;
 }
@@ -284,19 +285,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
 	ssize_t ret = 0;
 	struct hwrng *rng;
 
+	buf[0] = '\0';
 	err = mutex_lock_interruptible(&rng_mutex);
 	if (err)
 		return -ERESTARTSYS;
-	buf[0] = '\0';
 	list_for_each_entry(rng, &rng_list, list) {
 		strncat(buf, rng->name, PAGE_SIZE - ret - 1);
 		ret += strlen(rng->name);
 		strncat(buf, " ", PAGE_SIZE - ret - 1);
 		ret++;
 	}
+	mutex_unlock(&rng_mutex);
 	strncat(buf, "\n", PAGE_SIZE - ret - 1);
 	ret++;
-	mutex_unlock(&rng_mutex);
 
 	return ret;
 }

-- 
			Amos.