Re: [PATCH 2/5] hw_random: use reference counts on each struct hwrng.

Amos Kong
2014-09-18T12:22:55Z

On Thu, Sep 18, 2014 at 12:18:23PM +0930, Rusty Russell wrote:
> current_rng holds one reference, and we bump it every time we want
> to do a read from it.
>
> This means we only hold the rng_mutex to grab or drop a reference,
> so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
> block on read of /dev/hwrng.
>
> Using a kref is overkill (we're always under the rng_mutex), but
> a standard pattern.
>
> This also solves the problem that the hwrng_fillfn thread was
> accessing current_rng without a lock, which could change (eg. to NULL)
> underneath it.

Hi Rusty,

> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> ---
> drivers/char/hw_random/core.c | 135 ++++++++++++++++++++++++++++--------------
> include/linux/hw_random.h | 2 +
> 2 files changed, 94 insertions(+), 43 deletions(-)

...

> static int rng_dev_open(struct inode *inode, struct file *filp)
> {
> /* enforce read-only access to this chrdev */
> @@ -154,21 +202,22 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
> ssize_t ret = 0;
> int err = 0;
> int bytes_read, len;
> + struct hwrng *rng;
>
> while (size) {
> - if (mutex_lock_interruptible(&rng_mutex)) {
> - err = -ERESTARTSYS;
> + rng = get_current_rng();
> + if (IS_ERR(rng)) {
> + err = PTR_ERR(rng);
> goto out;
> }
> -
> - if (!current_rng) {
> + if (!rng) {
> err = -ENODEV;
> - goto out_unlock;
> + goto out;
> }
>
> mutex_lock(&reading_mutex);
> if (!data_avail) {
> - bytes_read = rng_get_data(current_rng, rng_buffer,
> + bytes_read = rng_get_data(rng, rng_buffer,
> rng_buffer_size(),
> !(filp->f_flags & O_NONBLOCK));
> if (bytes_read < 0) {
> @@ -200,7 +249,6 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
> ret += len;
> }
>
> - mutex_unlock(&rng_mutex);
> mutex_unlock(&reading_mutex);
>
> if (need_resched())
> @@ -210,15 +258,16 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
> err = -ERESTARTSYS;

We need put_rng() in this error path. Otherwise, unhotplug will hang
in the end of hwrng_unregister()

| /* Just in case rng is reading right now, wait. */
| wait_event(rng_done, atomic_read(&rng->ref.refcount) == 0);

Steps to reproduce the hang:
guest) # dd if=/dev/hwrng of=/dev/null
cancel dd process after 10 seconds
guest) # dd if=/dev/hwrng of=/dev/null &
hotunplug rng device from qemu monitor
result: device can't be removed (still can find in QEMU monitor)

diff --git a/drivers/char/hw_random/core.c
b/drivers/char/hw_random/core.c
index 96fa067..4e22d70 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -258,6 +258,7 @@ static ssize_t rng_dev_read(struct file *filp,
char __user *buf,

if (signal_pending(current)) {
err = -ERESTARTSYS;
+ put_rng(rng);
goto out;
}

> goto out;
> }
> +
> + put_rng(rng);
> }
> out:
> return ret ? : err;
> -out_unlock:
> - mutex_unlock(&rng_mutex);
> - goto out;
> +
> out_unlock_reading:
> mutex_unlock(&reading_mutex);
> - goto out_unlock;
> + put_rng(rng);
> + goto out;
> }
>
>
> @@ -257,8 +306,8 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
> err = hwrng_init(rng);
> if (err)
> break;
> - hwrng_cleanup(current_rng);
> - current_rng = rng;
> + drop_current_rng();
> + set_current_rng(rng);
> err = 0;
> break;
> }
> @@ -272,17 +321,15 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
> struct device_attribute *attr,
> char *buf)
> {
> - int err;
> ssize_t ret;
> - const char *name = "none";
> + struct hwrng *rng;
>
> - err = mutex_lock_interruptible(&rng_mutex);
> - if (err)
> - return -ERESTARTSYS;
> - if (current_rng)
> - name = current_rng->name;
> - ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> - mutex_unlock(&rng_mutex);
> + rng = get_current_rng();
> + if (IS_ERR(rng))
> + return PTR_ERR(rng);
> +
> + ret = snprintf(buf, PAGE_SIZE, "%s\n", rng ? rng->name : "none");
> + put_rng(rng);
>
> return ret;
> }
> @@ -357,12 +404,16 @@ static int hwrng_fillfn(void *unused)
> long rc;
>
> while (!kthread_should_stop()) {
> - if (!current_rng)
> + struct hwrng *rng;
> +
> + rng = get_current_rng();
> + if (IS_ERR(rng) || !rng)
> break;
> mutex_lock(&reading_mutex);
> - rc = rng_get_data(current_rng, rng_fillbuf,
> + rc = rng_get_data(rng, rng_fillbuf,
> rng_buffer_size(), 1);
> mutex_unlock(&reading_mutex);
> + put_rng(rng);

^^^
This put_rng() called a deadlock. I will describe in the bottom.

> if (rc <= 0) {
> pr_warn("hwrng: no data available\n");
> msleep_interruptible(10000);
> @@ -423,14 +474,13 @@ int hwrng_register(struct hwrng *rng)
> err = hwrng_init(rng);
> if (err)
> goto out_unlock;
> - current_rng = rng;
> + set_current_rng(rng);
> }
> err = 0;
> if (!old_rng) {
> err = register_miscdev();
> if (err) {
> - hwrng_cleanup(rng);
> - current_rng = NULL;
> + drop_current_rng();
> goto out_unlock;
> }
> }
> @@ -457,22 +507,21 @@ EXPORT_SYMBOL_GPL(hwrng_register);
>
> void hwrng_unregister(struct hwrng *rng)
> {
> - int err;
> -
> mutex_lock(&rng_mutex);
>
> list_del(&rng->list);
> if (current_rng == rng) {
> - hwrng_cleanup(rng);
> - if (list_empty(&rng_list)) {
> - current_rng = NULL;
> - } else {
> - current_rng = list_entry(rng_list.prev, struct hwrng, list);
> - err = hwrng_init(current_rng);
> - if (err)
> - current_rng = NULL;
> + drop_current_rng();
> + if (!list_empty(&rng_list)) {
> + struct hwrng *tail;
> +
> + tail = list_entry(rng_list.prev, struct hwrng, list);
> +
> + if (hwrng_init(tail) == 0)
> + set_current_rng(tail);
> }
> }
> +
> if (list_empty(&rng_list)) {
> unregister_miscdev();
> if (hwrng_fill)

hwrng_unregister() and put_rng() grab the lock, if hwrng_unregister()
takes the lock, hwrng_fillfn() will stay at put_rng() to wait the
lock.

Right now, thread_stop() is insider lock protection, but we try to
wake up the fillfn thread and wait for its completion.

| wake_up_process(k);
| wait_for_completion(&kthread->exited);

The solution is moving kthread_stop() outsider of lock protection.

@@ -524,11 +525,11 @@ void hwrng_unregister(struct hwrng *rng)

if (list_empty(&rng_list)) {
unregister_miscdev();
+ mutex_unlock(&rng_mutex);
if (hwrng_fill)
kthread_stop(hwrng_fill);
- }
-
- mutex_unlock(&rng_mutex);
+ } else
+ mutex_unlock(&rng_mutex);

/* Just in case rng is reading right now, wait. */
wait_event(rng_done, atomic_read(&rng->ref.refcount) == 0);

================
After applied my additional two fixes, both cating hung and hotunplug
issues were resolved.

| test 0:
| hotunplug rng device from qemu monitor
|
| test 1:
| guest) # dd if=/dev/hwrng of=/dev/null &
| hotunplug rng device from qemu monitor
|
| test 2:
| guest) # dd if=/dev/random of=/dev/null &
| hotunplug rng device from qemu monitor
|
| test 4:
| guest) # dd if=/dev/hwrng of=/dev/null &
| cat /sys/devices/virtual/misc/hw_random/rng_*
|
| test 5:
| guest) # dd if=/dev/hwrng of=/dev/null
| cancel dd process after 10 seconds
| guest) # dd if=/dev/hwrng of=/dev/null &
| hotunplug rng device from qemu monitor
|
| test 6:
| use a fifo as rng backend, execute test 0 ~ 5 with no input of fifo

Test are all passed :-)

I know you are going or you already started your holiday, I will post
a v2 with my additional patches.

Thanks, Amos

> diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
> index 914bb08cd738..c212e71ea886 100644
> --- a/include/linux/hw_random.h
> +++ b/include/linux/hw_random.h
> @@ -14,6 +14,7 @@
>
> #include <linux/types.h>
> #include <linux/list.h>
> +#include <linux/kref.h>
>
> /**
> * struct hwrng - Hardware Random Number Generator driver
> @@ -44,6 +45,7 @@ struct hwrng {
>
> /* internal. */
> struct list_head list;
> + struct kref ref;
> };
>
> /** Register a new Hardware Random Number Generator driver. */
> --
> 1.9.1