Expand the scope of the sysctllock sx lock to protect the sysctl tree itself.

Back in 1.1 of kern_sysctl.c the sysctl() routine wired the "old" userland
buffer for most sysctls (everything except kern.vnode.*).  I think to prevent
issues with wiring too much memory it used a 'memlock' to serialize all
sysctl(2) invocations, meaning that only one user buffer could be wired at
a time.  In 5.0 the 'memlock' was converted to an sx lock and renamed to
'sysctl lock'.  However, it still only served the purpose of serializing
sysctls to avoid wiring too much memory and didn't actually protect the
sysctl tree as its name suggested.  These changes expand the lock to actually
protect the tree.

Later on in 5.0, sysctl was changed to not wire buffers for requests by
default (sysctl_handle_opaque() will still wire buffers larger than a single
page, however).  As a result, user buffers are no longer wired as often.
However, many sysctl handlers still wire user buffers, so it is still
desirable to serialize userland sysctl requests.  Kernel sysctl requests
are allowed to run in parallel, however.

- Expose sysctl_lock()/sysctl_unlock() routines to exclusively lock the
  sysctl tree for a few places outside of kern_sysctl.c that manipulate
  the sysctl tree directly including the kernel linker and vfs_register().
- sysctl_register() and sysctl_unregister() require the caller to lock
  the sysctl lock using sysctl_lock() and sysctl_unlock().  The rest of
  the public sysctl API manage the locking internally.
- Add a locked variant of sysctl_remove_oid() for internal use so that
  external uses of the API do not need to be aware of locking requirements.
- The kernel linker no longer needs Giant when manipulating the sysctl
  tree.
- Add a missing break to the loop in vfs_register() so that we stop looking
  at the sysctl MIB once we have changed it.

MFC after:	1 month

1 files changed, 3 insertions, 0 deletions

diff --git a/sys/kern/vfs_init.c b/sys/kern/vfs_init.c
index 3ce65ed..4367b8c 100644
--- a/sys/kern/vfs_init.c
+++ b/sys/kern/vfs_init.c
@@ -165,12 +165,15 @@ vfs_register(struct vfsconf *vfc)
 	 * preserved by re-registering the oid after modifying its
 	 * number.
 	 */
+	sysctl_lock();
 	SLIST_FOREACH(oidp, &sysctl__vfs_children, oid_link)
 		if (strcmp(oidp->oid_name, vfc->vfc_name) == 0) {
 			sysctl_unregister_oid(oidp);
 			oidp->oid_number = vfc->vfc_typenum;
 			sysctl_register_oid(oidp);
+			break;
 		}
+	sysctl_unlock();
 
 	/*
 	 * Initialise unused ``struct vfsops'' fields, to use