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SubjectRe: [PATCH 1/2] Update woken requeued futex_q lock_ptr
Darren Hart wrote:
> 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.

In fact, this doesn't affect the handle_early_requeue_pi_wakeup() code
in the slightest. It only needs to hold a hb->lock (either one is
adequate) to ensure the requeue routine has completed. By changing the
q->lock_ptr of the waiter to hb2->lock we ensure the pi_state is
protected from concurrent access by futex_wait_requeue_pi() and new
contending threads.

Ingo, please apply to tip/urgent.

Thanks,

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


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