[PATCH] [1/4] /dev/random: Fix latency in rekeying sequence number

[PATCH] [1/4] /dev/random: Fix latency in rekeying sequence number

[Theodore Ts'o]

Based on reports from Ingo's Latency Tracer that the TCP sequence number
rekey code is causing latency problems, I've moved the sequence number
rekey to be done out of a workqueue.

patch-random-1-rekey-workqueue

--- random.c	2004/08/19 22:48:42	1.1
+++ random.c	2004/08/19 22:49:20	1.2
@@ -2246,30 +2246,35 @@
 static spinlock_t ip_lock = SPIN_LOCK_UNLOCKED;
 static unsigned int ip_cnt;
 
-static struct keydata *__check_and_rekey(time_t time)
+static void rekey_seq_generator(void *private_)
 {
 	struct keydata *keyptr;
+	struct timeval 	tv;
+
+	do_gettimeofday(&tv);
+
 	spin_lock_bh(&ip_lock);
 	keyptr = &ip_keydata[ip_cnt&1];
-	if (!keyptr->rekey_time || (time - keyptr->rekey_time) > REKEY_INTERVAL) {
-		keyptr = &ip_keydata[1^(ip_cnt&1)];
-		keyptr->rekey_time = time;
-		get_random_bytes(keyptr->secret, sizeof(keyptr->secret));
-		keyptr->count = (ip_cnt&COUNT_MASK)<<HASH_BITS;
-		mb();
-		ip_cnt++;
-	}
+
+	keyptr = &ip_keydata[1^(ip_cnt&1)];
+	keyptr->rekey_time = tv.tv_sec;
+	get_random_bytes(keyptr->secret, sizeof(keyptr->secret));
+	keyptr->count = (ip_cnt&COUNT_MASK)<<HASH_BITS;
+	mb();
+	ip_cnt++;
+
 	spin_unlock_bh(&ip_lock);
-	return keyptr;
 }
 
+static DECLARE_WORK(rekey_work, rekey_seq_generator, NULL);
+
 static inline struct keydata *check_and_rekey(time_t time)
 {
 	struct keydata *keyptr = &ip_keydata[ip_cnt&1];
 
 	rmb();
 	if (!keyptr->rekey_time || (time - keyptr->rekey_time) > REKEY_INTERVAL) {
-		keyptr = __check_and_rekey(time);
+		schedule_work(&rekey_work);
 	}
 
 	return keyptr;

Re: [PATCH] [1/4] /dev/random: Fix latency in rekeying sequence number

[Lee Revell]

On Fri, 2004-08-20 at 00:57, Theodore Ts'o wrote:
> Based on reports from Ingo's Latency Tracer that the TCP sequence number
> rekey code is causing latency problems, I've moved the sequence number
> rekey to be done out of a workqueue.
> 

This patch does not actually fix the problem, as 3-700usecs is still
spent in the spinlocked region, this just causes it to be done out of a
workqueue.  It reduces the incidence of problems, but for latency
sensitive applications the worst-case is the important one.

Lee

Re: [PATCH] [1/4] /dev/random: Fix latency in rekeying sequence number

[Jens Maurer]

Lee Revell wrote:
> This patch does not actually fix the problem, as 3-700usecs is still
> spent in the spinlocked region, this just causes it to be done out of a
> workqueue.

May I suggest that get_random_bytes() be called *outside* of the
spinlocked region on a temporary buffer, assuming that the SHA
algorithm is the problem here?

Something like this (untested):

   u32 tmp[sizeof(keyptr->secret)/4];      /* on stack */
   get_random_bytes(tmp, sizeof(tmp));
   spin_lock_bh(&ip_lock);
   memcpy(keyptr->secret, tmp, sizeof(keyptr->secret));
   /* ... other housekeeping ... */
   spin_unlock_bh(&ip_lock);

Jens Maurer