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SubjectRe: [patch 58/63] futex: Prevent requeue_pi() lock nesting issue on RT
On Mon, Aug 02, 2021 at 02:56:09PM +0200, Peter Zijlstra wrote:
> static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
> {
> + int old, new;
>
> + old = atomic_read_acquire(&q->requeue_state);
> + do {

if (old == Q_REQUEUE_PI_IGNORE)
return;

> if (locked >= 0) {

WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
old != Q_REQUEUE_PI_WAIT)

> /* Requeue succeeded. Set DONE or LOCKED */
> new = Q_REQUEUE_PI_DONE + locked;
> + } else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
> /* Deadlock, no early wakeup interleave */
> new = Q_REQUEUE_PI_NONE;
> } else {

WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);

> /* Deadlock, early wakeup interleave. */
> new = Q_REQUEUE_PI_IGNORE;
> }
> + } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
>
> #ifdef CONFIG_PREEMPT_RT
> /* If the waiter interleaved with the requeue let it know */
> + if (unlikely(old == Q_REQUEUE_PI_WAIT))
> rcuwait_wake_up(&q->requeue_wait);
> #endif
> }

I think...

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