I am trying to walk a wait_queue_t list as part of a LSM file_permission function. The purpose is to act on each task which has blocked while trying to read from a pipe. I have modeled my code on __wake_up_common() in kernel/sched/core.c, and it looks something like this: if (i_pipe->reader <= 0) { return; } list_for_each_entry_safe(curr, next, &i_pipe->wait.task, task_list) { [...] struct task_struct *blocked = curr->private; [...] } I have tried to wrap this with: spin_lock_irqsave(&i_pipe->wait.lock, flags) spin_unlock_irqrestore[...] and also: write_lock_irq(&tasklist_lock). write_unlock_irq[...] Despite this, I sometimes find that blocked (AKA curr->private) == NULL during an iteration of the list_for_each_entry_safe loop, and this surprises me. Why would there be en entry in the wait_queue_t list which does not have a process associated with it? Is the data structure moving out from under me? Is there something else I should lock? Thank you, -- Mike :wq