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;
}
(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.
On (Tue) 09 Sep 2014 [23:23:07], Amos Kong wrote:> (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.How common is it going to be to have a long-running 'dd' process on /dev/hwrng? Also, with the new khwrng thread, reading from /dev/hwrng isn't required -- just use /dev/random? (This doesn't mean we shouldn't fix the issue here...)> 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);We have a need_resched() right below. Why doesn't that work?> 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;The following hunk doesn't work:> + err = -ENODEV; > err = mutex_lock_interruptible(&rng_mutex);err is being set to another value in the next line!> if (err) > return -ERESTARTSYS; > - err = -ENODEV;And all usage of err below now won't have -ENODEV but some other value.> 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);This looks OK...> > 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);But this isn't resulting in savings; the majority of the time is being spent in the for loop, and that writes to the buffer. BTW I don't expect strcat'ing to the buf in each of these scenarios is a long operation, so this reworking doesn't strike to me as something we should pursue. Amit