On Wed, Sep 10, 2014 at 11:22:12AM +0530, Amit Shah wrote:
> 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?
Not a common usage, but we have this strict testing.
> Also, with the new khwrng thread, reading from /dev/hwrng isn't
> required -- just use /dev/random?
Yes.
> (This doesn't mean we shouldn't fix the issue here...)
Completely agree :-)
> > 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?
need_resched() is giving chance for userspace to
> > if (need_resched())
It never success in my debugging.
If we remove this check and always call schedule_timeout_interruptible(1),
problem also disappears.
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index aa30a25..263a370 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -195,8 +195,7 @@ static ssize_t rng_dev_read(struct file *filp,
char __user *buf,
mutex_unlock(&rng_mutex);
- if (need_resched())
- schedule_timeout_interruptible(1);
+ schedule_timeout_interruptible(1);
if (signal_pending(current)) {
err = -ERESTARTSYS;
> > 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.
Oops!
> > 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.
Right
> 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
--
Amos.
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