From 3e829b18d6d3f8b4290538c7c76dc34212603852 Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@FreeBSD.org>
Date: Thu, 14 May 2009 22:01:32 +0000
Subject: [PATCH] - Use a separate sx lock to try to limit the number of
 concurrent userland   sysctl requests to avoid wiring too much user memory. 
 Only grab this   lock if the user's old buffer is larger than a page as a
 tradeoff to   allow more concurrency for common small requests. - Just use a
 shared lock on the sysctl tree for user sysctl requests now.

MFC after:	1 week
---
 sys/kern/kern_sysctl.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/sys/kern/kern_sysctl.c b/sys/kern/kern_sysctl.c
index 89fbb9ab3ee0..bf539bea3bc7 100644
--- a/sys/kern/kern_sysctl.c
+++ b/sys/kern/kern_sysctl.c
@@ -77,11 +77,12 @@ static MALLOC_DEFINE(M_SYSCTLTMP, "sysctltmp", "sysctl temp output buffer");
  * API rather than using the dynamic API.  Use of the dynamic API is
  * strongly encouraged for most code.
  *
- * This lock is also used to serialize userland sysctl requests.  Some
- * sysctls wire user memory, and serializing the requests limits the
- * amount of wired user memory in use.
+ * The sysctlmemlock is used to limit the amount of user memory wired for
+ * sysctl requests.  This is implemented by serializing any userland
+ * sysctl requests larger than a single page via an exclusive lock.
  */
 static struct sx sysctllock;
+static struct sx sysctlmemlock;
 
 #define	SYSCTL_SLOCK()		sx_slock(&sysctllock)
 #define	SYSCTL_SUNLOCK()	sx_sunlock(&sysctllock)
@@ -543,6 +544,7 @@ sysctl_register_all(void *arg)
 {
 	struct sysctl_oid **oidp;
 
+	sx_init(&sysctlmemlock, "sysctl mem");
 	SYSCTL_INIT();
 	SYSCTL_XLOCK();
 	SET_FOREACH(oidp, sysctl_set)
@@ -1565,7 +1567,7 @@ userland_sysctl(struct thread *td, int *name, u_int namelen, void *old,
     size_t *oldlenp, int inkernel, void *new, size_t newlen, size_t *retval,
     int flags)
 {
-	int error = 0;
+	int error = 0, memlocked;
 	struct sysctl_req req;
 
 	bzero(&req, sizeof req);
@@ -1605,14 +1607,20 @@ userland_sysctl(struct thread *td, int *name, u_int namelen, void *old,
 	if (KTRPOINT(curthread, KTR_SYSCTL))
 		ktrsysctl(name, namelen);
 #endif
-	
-	SYSCTL_XLOCK();
+
+	if (req.oldlen > PAGE_SIZE) {
+		memlocked = 1;
+		sx_xlock(&sysctlmemlock);
+	} else
+		memlocked = 0;
 	CURVNET_SET(TD_TO_VNET(curthread));
 
 	for (;;) {
 		req.oldidx = 0;
 		req.newidx = 0;
+		SYSCTL_SLOCK();
 		error = sysctl_root(0, name, namelen, &req);
+		SYSCTL_SUNLOCK();
 		if (error != EAGAIN)
 			break;
 		uio_yield();
@@ -1622,7 +1630,8 @@ userland_sysctl(struct thread *td, int *name, u_int namelen, void *old,
 
 	if (req.lock == REQ_WIRED && req.validlen > 0)
 		vsunlock(req.oldptr, req.validlen);
-	SYSCTL_XUNLOCK();
+	if (memlocked)
+		sx_xunlock(&sysctlmemlock);
 
 	if (error && error != ENOMEM)
 		return (error);