• Oleg Nesterov's avatar
    introduce for_each_thread() to replace the buggy while_each_thread() · 0c740d0a
    Oleg Nesterov authored
    while_each_thread() and next_thread() should die, almost every lockless
    usage is wrong.
    
    1. Unless g == current, the lockless while_each_thread() is not safe.
    
       while_each_thread(g, t) can loop forever if g exits, next_thread()
       can't reach the unhashed thread in this case. Note that this can
       happen even if g is the group leader, it can exec.
    
    2. Even if while_each_thread() itself was correct, people often use
       it wrongly.
    
       It was never safe to just take rcu_read_lock() and loop unless
       you verify that pid_alive(g) == T, even the first next_thread()
       can point to the already freed/reused memory.
    
    This patch adds signal_struct->thread_head and task->thread_node to
    create the normal rcu-safe list with the stable head.  The new
    for_each_thread(g, t) helper is always safe under rcu_read_lock() as
    long as this task_struct can't go away.
    
    Note: of course it is ugly to have both task_struct->thread_node and the
    old task_struct->thread_group, we will kill it later, after we change
    the users of while_each_thread() to use for_each_thread().
    
    Perhaps we can kill it even before we convert all users, we can
    reimplement next_thread(t) using the new thread_head/thread_node.  But
    we can't do this right now because this will lead to subtle behavioural
    changes.  For example, do/while_each_thread() always sees at least one
    task, while for_each_thread() can do nothing if the whole thread group
    has died.  Or thread_group_empty(), currently its semantics is not clear
    unless thread_group_leader(p) and we need to audit the callers before we
    can change it.
    
    So this patch adds the new interface which has to coexist with the old
    one for some time, hopefully the next changes will be more or less
    straightforward and the old one will go away soon.
    Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
    Reviewed-by: default avatarSergey Dyasly <dserrg@gmail.com>
    Tested-by: default avatarSergey Dyasly <dserrg@gmail.com>
    Reviewed-by: default avatarSameer Nanda <snanda@chromium.org>
    Acked-by: default avatarDavid Rientjes <rientjes@google.com>
    Cc: "Eric W. Biederman" <ebiederm@xmission.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Mandeep Singh Baines <msb@chromium.org>
    Cc: "Ma, Xindong" <xindong.ma@intel.com>
    Cc: Michal Hocko <mhocko@suse.cz>
    Cc: "Tu, Xiaobing" <xiaobing.tu@intel.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0c740d0a
exit.c 42.6 KB