Commented SlotStr with information what what locks protect what.

security/nss/lib/softoken/pkcs11i.h

@@ -341,6 +341,13 @@ struct PK11SessionStr {
 
 /*
  * slots (have sessions and objects)
+ *
+ * The array of sessionLock's protect the session hash table (head[])
+ * as well as the reference count of session objects in that bucket
+ * (head[]->refCount),  objectLock protects all elements of the token
+ * object hash table (tokObjects[]), and slotLock protects the
+ * remaining protected elements:  password, isLoggedIn, ssoLoggedIn,
+ * sessionIDCount, sessionCount, and rwSessionCount.
  */
 struct PK11SlotStr {
     CK_SLOT_ID		slotID;