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.
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