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. -- 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
