linux-kernel - Re: [PATCH 1/2] Update woken requeued futex_q lock

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Message-ID: <4A7A66D7.3090203@us.ibm.com>
Date:	Wed, 05 Aug 2009 22:15:03 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
CC:	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	mingo@...e.hu, dino@...ibm.com, johnstul@...ibm.com,
	John Kacur <jkacur@...hat.com>
Subject: Re: [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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