• Oleg Nesterov's avatar
    hung_task: fix the broken rcu_lock_break() logic · 6027ce49
    Oleg Nesterov authored
    check_hung_uninterruptible_tasks()->rcu_lock_break() introduced by
    "softlockup: check all tasks in hung_task" commit ce9dbe24 looks
    absolutely wrong.
    
    	- rcu_lock_break() does put_task_struct(). If the task has exited
    	  it is not safe to even read its ->state, nothing protects this
    	  task_struct.
    
    	- The TASK_DEAD checks are wrong too. Contrary to the comment, we
    	  can't use it to check if the task was unhashed. It can be unhashed
    	  without TASK_DEAD, or it can be valid with TASK_DEAD.
    
    	  For example, an autoreaping task can do release_task(current)
    	  long before it sets TASK_DEAD in do_exit().
    
    	  Or, a zombie task can have ->state == TASK_DEAD but release_task()
    	  was not called, and in this case we must not break the loop.
    
    Change this code to check pid_alive() instead, and do this before we drop
    the reference to the task_struct.
    
    Note: while_each_thread() under rcu_read_lock() is not really safe, it can
    livelock.  This will be fixed later, but fortunately in this case the
    "max_count" logic saves us anyway.
    Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
    Acked-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
    Acked-by: default avatarMandeep Singh Baines <msb@google.com>
    Acked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    6027ce49
hung_task.c 5.22 KB