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SubjectRe: [PATCH 1/2] Update woken requeued futex_q lock_ptr
Darren Hart wrote:
> futex_requeue() can acquire the lock on behalf of a waiter during the
> requeue
> loop in the event of a lock steal or owner died. futex_wait_requeue_pi()
> cleans
> up the pi_state owner, using the lock_ptr to protect against concurrent
> access
> to the pi_state. The pi_state is found on the requeue target futex hash
> bucket
> so the lock_ptr needs to be updated accordingly. The problem manifested by
> triggering the WARN_ON in lookup_pi_state() about the pid !=
> pi_state->owner
> pid.
> The astute reviewer will note that still exists a race between the time
> futex_requeue() releases hb2->lock() and the time when
> futex_wait_requeue_pi()
> acquires it. During this time the pi_state and the futex uaddr are not
> in sync
> with the rt_mutex ownership. This patch closes the window to the point
> where
> my tests now pass, but we still need to address it.
>
> Note: Please apply to mainline and rt
>


> static inline
> -void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
> +void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
> + struct futex_hash_bucket *hb)
> {
> drop_futex_key_refs(&q->key);
> get_futex_key_refs(key);
> q->key = *key;
> + q->lock_ptr = &hb->lock;

Hrm... turns out changing this breaks the
handle_early_requeue_pi_wakeup() logic. I'll have to respin this patch
to account for that as well. Please hold off on this patch.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team


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