Function vfs_busyfs

freebsd/kern/vfs_subr.c:865–907

* Lookup a mount point by filesystem identifier, busying it before * returning. * * To avoid congestion on mountlist_mtx, implement simple direct-mapped * cache for popular filesystem identifiers. The cache is lockess, using * the fact that struct mount's are never freed. In worst case we may * get pointer to unmounted or even different filesystem, so we have to * check what we got, and g

 * check what we got, and go slow way if so.
 */
struct mount *
vfs_busyfs(fsid_t *fsid)
{
#define	FSID_CACHE_SIZE	256
	typedef struct mount * volatile vmp_t;
	static vmp_t cache[FSID_CACHE_SIZE];
	struct mount *mp;
	int error;
	uint32_t hash;

	CTR2(KTR_VFS, "%s: fsid %p", __func__, fsid);
	hash = fsid->val[0] ^ fsid->val[1];
	hash = (hash >> 16 ^ hash) & (FSID_CACHE_SIZE - 1);
	mp = cache[hash];
	if (mp == NULL || fsidcmp(&mp->mnt_stat.f_fsid, fsid) != 0)
		goto slow;
	if (vfs_busy(mp, 0) != 0) {
		cache[hash] = NULL;
		goto slow;
	}
	if (fsidcmp(&mp->mnt_stat.f_fsid, fsid) == 0)
		return (mp);
	else
	    vfs_unbusy(mp);

slow:
	mtx_lock(&mountlist_mtx);
	TAILQ_FOREACH(mp, &mountlist, mnt_list) {
		if (fsidcmp(&mp->mnt_stat.f_fsid, fsid) == 0) {
			error = vfs_busy(mp, MBF_MNTLSTLOCK);
			if (error) {
				cache[hash] = NULL;
				mtx_unlock(&mountlist_mtx);
				return (NULL);
			}
			cache[hash] = mp;
			return (mp);
		}
	}
	CTR2(KTR_VFS, "%s: lookup failed for %p id", __func__, fsid);
	mtx_unlock(&mountlist_mtx);
	return ((struct mount *) 0);
}

/*
 * Check if a user can access privileged mount options.