• Ying Xue's avatar
    tipc: remove 'links' list from tipc_bearer struct · c61dd61d
    Ying Xue authored
    In our ongoing effort to simplify the TIPC locking structure,
    we see a need to remove the linked list for tipc_links
    in the bearer. This can be explained as follows.
    
    Currently, we have three different ways to access a link,
    via three different lists/tables:
    
    1: Via a node hash table:
       Used by the time-critical outgoing/incoming data paths.
       (e.g. link_send_sections_fast() and tipc_recv_msg() ):
    
    grab net_lock(read)
       find node from node hash table
       grab node_lock
           select link
           grab bearer_lock
              send_msg()
           release bearer_lock
       release node lock
    release net_lock
    
    2: Via a global linked list for nodes:
       Used by configuration commands (link_cmd_set_value())
    
    grab net_lock(read)
       find node and link from global node list (using link name)
       grab node_lock
           update link
       release node lock
    release net_lock
    
    (Same locking order as above. No problem.)
    
    3: Via the bearer's linked link list:
       Used by notifications from interface (e.g. tipc_disable_bearer() )
    
    grab net_lock(write)
       grab bearer_lock
          get link ptr from bearer's link list
          get node from link
          grab node_lock
             delete link
          release node lock
       release bearer_lock
    release net_lock
    
    (Different order from above, but works because we grab the
    outer net_lock in write mode first, excluding all other access.)
    
    The first major goal in our simplification effort is to get rid
    of the "big" net_lock, replacing it with rcu-locks when accessing
    the node list and node hash array. This will come in a later patch
    series.
    
    But to get there we first need to rewrite access methods ##2 and 3,
    since removal of net_lock would introduce three major problems:
    
    a) In access method #2, we access the link before taking the
       protecting node_lock. This will not work once net_lock is gone,
       so we will have to change the access order. We will deal with
       this in a later commit in this series, "tipc: add node lock
       protection to link found by link_find_link()".
    
    b) When the outer protection from net_lock is gone, taking
       bearer_lock and node_lock in opposite order of method 1) and 2)
       will become an obvious deadlock hazard. This is fixed in the
       commit ("tipc: remove bearer_lock from tipc_bearer struct")
       later in this series.
    
    c) Similar to what is described in problem a), access method #3
       starts with using a link pointer that is unprotected by node_lock,
       in order to via that pointer find the correct node struct and
       lock it. Before we remove net_lock, this access order must be
       altered. This is what we do with this commit.
    
    We can avoid introducing problem problem c) by even here using the
    global node list to find the node, before accessing its links. When
    we loop though the node list we use the own bearer identity as search
    criteria, thus easily finding the links that are associated to the
    resetting/disabling bearer. It should be noted that although this
    method is somewhat slower than the current list traversal, it is in
    no way time critical. This is only about resetting or deleting links,
    something that must be considered relatively infrequent events.
    
    As a bonus, we can get rid of the mutual pointers between links and
    bearers. After this commit, pointer dependency go in one direction
    only: from the link to the bearer.
    
    This commit pre-empts introduction of problem c) as described above.
    Signed-off-by: default avatarYing Xue <ying.xue@windriver.com>
    Reviewed-by: default avatarPaul Gortmaker <paul.gortmaker@windriver.com>
    Signed-off-by: default avatarJon Maloy <jon.maloy@ericsson.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    c61dd61d
bearer.c 15.9 KB