• Pavel Emelyanov's avatar
    Make access to task's nsproxy lighter · cf7b708c
    Pavel Emelyanov authored
    When someone wants to deal with some other taks's namespaces it has to lock
    the task and then to get the desired namespace if the one exists.  This is
    slow on read-only paths and may be impossible in some cases.
    
    E.g.  Oleg recently noticed a race between unshare() and the (sent for
    review in cgroups) pid namespaces - when the task notifies the parent it
    has to know the parent's namespace, but taking the task_lock() is
    impossible there - the code is under write locked tasklist lock.
    
    On the other hand switching the namespace on task (daemonize) and releasing
    the namespace (after the last task exit) is rather rare operation and we
    can sacrifice its speed to solve the issues above.
    
    The access to other task namespaces is proposed to be performed
    like this:
    
         rcu_read_lock();
         nsproxy = task_nsproxy(tsk);
         if (nsproxy != NULL) {
                 / *
                   * work with the namespaces here
                   * e.g. get the reference on one of them
                   * /
         } / *
             * NULL task_nsproxy() means that this task is
             * almost dead (zombie)
             * /
         rcu_read_unlock();
    
    This patch has passed the review by Eric and Oleg :) and,
    of course, tested.
    
    [clg@fr.ibm.com: fix unshare()]
    [ebiederm@xmission.com: Update get_net_ns_by_pid]
    Signed-off-by: default avatarPavel Emelyanov <xemul@openvz.org>
    Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
    Cc: Oleg Nesterov <oleg@tv-sign.ru>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Serge Hallyn <serue@us.ibm.com>
    Signed-off-by: default avatarCedric Le Goater <clg@fr.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    cf7b708c
fork.c 41.6 KB