Mark most often used sysctl's as MPSAFE.

After running a `make buildkernel', I noticed most of the Giant locks in
sysctl are only caused by a very small amount of sysctl's:

- sysctl.name2oid. This one is locked by SYSCTL_LOCK, just like
  sysctl.oidfmt.

- kern.ident, kern.osrelease, kern.version, etc. These are just constant
  strings.

- kern.arandom, used by the stack protector. It is already protected by
  arc4_mtx.

I also saw the following sysctl's show up. Not as often as the ones
above, but still quite often:

- security.jail.jailed. Also mark security.jail.list as MPSAFE. They
  don't need locking or already use allprison_lock.

- kern.devname, used by devname(3), ttyname(3), etc.

This seems to reduce Giant locking inside sysctl by ~75% in my primitive
test setup.

1 files changed, 3 insertions, 2 deletions

diff --git a/sys/fs/devfs/devfs_devs.c b/sys/fs/devfs/devfs_devs.c
index 028f5c9..4041911 100644
--- a/sys/fs/devfs/devfs_devs.c
+++ b/sys/fs/devfs/devfs_devs.c
@@ -103,8 +103,9 @@ sysctl_devname(SYSCTL_HANDLER_ARGS)
 	return (error);
 }
 
-SYSCTL_PROC(_kern, OID_AUTO, devname, CTLTYPE_OPAQUE|CTLFLAG_RW|CTLFLAG_ANYBODY,
-	NULL, 0, sysctl_devname, "", "devname(3) handler");
+SYSCTL_PROC(_kern, OID_AUTO, devname,
+    CTLTYPE_OPAQUE|CTLFLAG_RW|CTLFLAG_ANYBODY|CTLFLAG_MPSAFE,
+    NULL, 0, sysctl_devname, "", "devname(3) handler");
 
 SYSCTL_INT(_debug_sizeof, OID_AUTO, cdev, CTLFLAG_RD,
     0, sizeof(struct cdev), "sizeof(struct cdev)");