Re: [PATCH 6/7] random: make /dev/urandom scalable for silly userspace programs

[Theodore Ts'o]

On Sun, Aug 21, 2016 at 12:53:15PM +0300, Jan Varho wrote:
> On Mon, Jun 13, 2016 at 6:48 PM, Theodore Ts'o  wrote:
> > +static inline void maybe_reseed_primary_crng(void)
> > +{
> > +   if (crng_init > 2 &&
> > +   time_after(jiffies, primary_crng.init_time + 
> > CRNG_RESEED_INTERVAL))
> > +   crng_reseed(_crng, _pool);
> > +}
> 
> Is the above function (which is now in 4.8-rc2) supposed to do
> something? It seems to have no callers and the maximum value of
> crng_init is 2.

It's dead code.  Its function got moved into _extra_crng(), and you're
right, these days crng_init never gets above 2.  Thanks for pointing
that out.  I'll take it out as a cleanup patch.

 - Ted
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Re: [PATCH 6/7] random: make /dev/urandom scalable for silly userspace programs

[Jan Varho]

On Mon, Jun 13, 2016 at 6:48 PM, Theodore Ts'o  wrote:
> +static inline void maybe_reseed_primary_crng(void)
> +{
> +   if (crng_init > 2 &&
> +   time_after(jiffies, primary_crng.init_time + 
> CRNG_RESEED_INTERVAL))
> +   crng_reseed(_crng, _pool);
> +}

Hi,

Is the above function (which is now in 4.8-rc2) supposed to do
something? It seems to have no callers and the maximum value of
crng_init is 2.
--
Jan Varho
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[PATCH 6/7] random: make /dev/urandom scalable for silly userspace programs

[Theodore Ts'o]

On a system with a 4 socket (NUMA) system where a large number of
application threads were all trying to read from /dev/urandom, this
can result in the system spending 80% of its time contending on the
global urandom spinlock.  The application should have used its own
PRNG, but let's try to help it from running, lemming-like, straight
over the locking cliff.

Reported-by: Andi Kleen 
Signed-off-by: Theodore Ts'o 
---
 drivers/char/random.c | 62 +++
 1 file changed, 58 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 841f9a8..d640865 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -434,6 +434,8 @@ struct crng_state primary_crng = {
  */
 static int crng_init = 0;
 #define crng_ready() (likely(crng_init > 0))
+static void _extract_crng(struct crng_state *crng,
+ __u8 out[CHACHA20_BLOCK_SIZE]);
 static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE]);
 static void process_random_ready_list(void);
 
@@ -754,6 +756,16 @@ static void credit_entropy_bits_safe(struct entropy_store 
*r, int nbits)
 
 static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 
+#ifdef CONFIG_NUMA
+/*
+ * Hack to deal with crazy userspace progams when they are all trying
+ * to access /dev/urandom in parallel.  The programs are almost
+ * certainly doing something terribly wrong, but we'll work around
+ * their brain damage.
+ */
+static struct crng_state **crng_node_pool __read_mostly;
+#endif
+
 static void crng_initialize(struct crng_state *crng)
 {
int i;
@@ -815,7 +827,7 @@ static void crng_reseed(struct crng_state *crng, struct 
entropy_store *r)
if (num == 0)
return;
} else
-   extract_crng(buf.block);
+   _extract_crng(_crng, buf.block);
spin_lock_irqsave(_crng.lock, flags);
for (i = 0; i < 8; i++) {
unsigned long   rv;
@@ -835,19 +847,26 @@ static void crng_reseed(struct crng_state *crng, struct 
entropy_store *r)
spin_unlock_irqrestore(_crng.lock, flags);
 }
 
+static inline void maybe_reseed_primary_crng(void)
+{
+   if (crng_init > 2 &&
+   time_after(jiffies, primary_crng.init_time + CRNG_RESEED_INTERVAL))
+   crng_reseed(_crng, _pool);
+}
+
 static inline void crng_wait_ready(void)
 {
wait_event_interruptible(crng_init_wait, crng_ready());
 }
 
-static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
+static void _extract_crng(struct crng_state *crng,
+ __u8 out[CHACHA20_BLOCK_SIZE])
 {
unsigned long v, flags;
-   struct crng_state *crng = _crng;
 
if (crng_init > 1 &&
time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL))
-   crng_reseed(crng, _pool);
+   crng_reseed(crng, crng == _crng ? _pool : NULL);
spin_lock_irqsave(>lock, flags);
if (arch_get_random_long())
crng->state[14] ^= v;
@@ -857,6 +876,19 @@ static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
spin_unlock_irqrestore(>lock, flags);
 }
 
+static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
+{
+   struct crng_state *crng = NULL;
+
+#ifdef CONFIG_NUMA
+   if (crng_node_pool)
+   crng = crng_node_pool[numa_node_id()];
+   if (crng == NULL)
+#endif
+   crng = _crng;
+   _extract_crng(crng, out);
+}
+
 static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
 {
ssize_t ret = 0, i;
@@ -1575,9 +1607,31 @@ static void init_std_data(struct entropy_store *r)
  */
 static int rand_initialize(void)
 {
+#ifdef CONFIG_NUMA
+   int i;
+   int num_nodes = num_possible_nodes();
+   struct crng_state *crng;
+   struct crng_state **pool;
+#endif
+
init_std_data(_pool);
init_std_data(_pool);
crng_initialize(_crng);
+
+#ifdef CONFIG_NUMA
+   pool = kmalloc(num_nodes * sizeof(void *),
+  GFP_KERNEL|__GFP_NOFAIL|__GFP_ZERO);
+   for (i=0; i < num_nodes; i++) {
+   crng = kmalloc_node(sizeof(struct crng_state),
+   GFP_KERNEL | __GFP_NOFAIL, i);
+   spin_lock_init(>lock);
+   crng_initialize(crng);
+   pool[i] = crng;
+
+   }
+   mb();
+   crng_node_pool = pool;
+#endif
return 0;
 }
 early_initcall(rand_initialize);
-- 
2.5.0

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