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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
