Re: lockdep trace from posix timers

[Oleg Nesterov]

On 08/16, Peter Zijlstra wrote:
>
>      write_lock_irq(&tasklist_lock)
>      task_lock(parent)		parent->alloc_lock

And this is already wrong. See the comment above task_lock().

> And since it_lock is IRQ-safe and alloc_lock isn't, you've got the IRQ
> inversion deadlock reported.

Yes. Or, IOW, write_lock(tasklist) is IRQ-safe and thus it can't nest
with alloc_lock.

> David, Al, anybody want to have a go at fixing this?

I still think that task_work_add() should synhronize with exit_task_work()
itself and fail if necessary. But I wasn't able to convince Al ;)

Oleg.

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