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

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.

                        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

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

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