Re: lockdep trace from posix timers

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2012-08-28 at 19:01 +0200, Oleg Nesterov wrote:
> >  struct callback_head *
> >  task_work_cancel(struct task_struct *task, task_work_func_t func)
> >  {
> > +     struct callback_head **workp, *work;
> > +
> > +again:
> > +     workp = &task->task_works;
> > +     work = *workp;
> > +     while (work) {
> > +             if (work->func == func) {
> 
> But you can't dereference this pointer. Without some locking this
> can race with another task_work_cancel() or task_work_run(), this
> work can be free/unmapped/reused.
> 
> > +                     if (cmpxchg(workp, work, work->next) == work)
> > +                             return work;
> 
> Or this can race with task_work_cancel(work) + task_work_add(work).
> cmpxchg() can succeed even if work->func is already different. 

Bah.. you and your races ;-)

Surely we can do this locklessly.. I'll go try harder still.