- 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

1 files changed, 16 insertions, 7 deletions

diff --git a/sys/kern/kern_sysctl.c b/sys/kern/kern_sysctl.c
index 89fbb9a..bf539be 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);