Re: lockdep trace from posix timers

[Oleg Nesterov]

On 08/28, Oleg Nesterov wrote:
>
> On 08/28, Peter Zijlstra wrote:
> >
> > +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.

Even simpler, this can race with another task_work_cancel() which
is going to remove work->next.

Oleg.

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