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SubjectRe: [PATCH 1/4 V2 ] futex,plist: pass the real head of the priority list to plist_del()
On 12/21/2010 01:55 AM, Lai Jiangshan wrote:

Hi Lai,

Looks about ready to me, only a couple of more nitpics from me.

>
> Some plist_del()s in kernel/futex.c are passed a faked head of the
> priority list.
>
> It can work because current code does not require the real head

Awkward, how about:
It does not fail because the current...

> in plist_del(). The code of plist_del() just uses the head for checking,
> so it will not cause bad result even when we use a faked head.

a bad result

>
> But it is an undocumented usage:

s/an//
>
> /**
> * plist_del - Remove a @node from plist.
> *
> * @node: &struct plist_node pointer - entry to be removed
> * @head: &struct plist_head pointer - list head
> */
>
> The document said that @head is "list head" the head of the priority list.
>
> In futex code, several places use "plist_del(&q->list,&q->list.plist);",
> they passes faked head, we fix them all.
>
> Thank to Darren Hart for many suggests.

s/Thank/Thanks/
s/suggests/suggestions/

>
> Signed-off-by: Lai Jiangshan<laijs@cn.fujitsu.com>
> ---
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 3019b92..d901f40 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -740,6 +740,23 @@ retry:
> return ret;
> }
>
> +/**
> + * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
> + * @q: The futex_q to unqueue
> + *
> + * The q->lock_ptr must not be NULL and must be held by the caller.
> + */
> +static void __unqueue_futex(struct futex_q *q)
> +{
> + struct futex_hash_bucket *hb;
> +
> + if (WARN_ON(!q->lock_ptr || !spin_is_locked(q->lock_ptr)))
> + return;
> +
> + hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
> + plist_del(&q->list,&hb->chain);
> +}

I like this approach better than the previous version.

> +
> /*
> * The hash bucket lock must be held when this is called.
> * Afterwards, the futex_q must not be accessed.
> @@ -757,7 +774,7 @@ static void wake_futex(struct futex_q *q)
> */
> get_task_struct(p);
>
> - plist_del(&q->list,&q->list.plist);
> + __unqueue_futex(q);
> /*
> * The waiting task can free the futex_q as soon as
> * q->lock_ptr = NULL is written, without taking any locks. A
> @@ -1067,7 +1084,7 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
> q->key = *key;
>
> WARN_ON(plist_node_empty(&q->list));
> - plist_del(&q->list,&q->list.plist);
> + __unqueue_futex(q);

The WARN_ON here is used several times, but there is no longer an
explicit plist operation to follow. Suggest moving the WARN_ON into
__unqueue_futex() and keep it together with the plist_del.


> WARN_ON(!q->rt_waiter);
> q->rt_waiter = NULL;
> @@ -1471,7 +1488,7 @@ retry:
> goto retry;
> }
> WARN_ON(plist_node_empty(&q->list));
> - plist_del(&q->list,&q->list.plist);
> + __unqueue_futex(q);

here too

>
> BUG_ON(q->pi_state);
>
> @@ -1492,7 +1509,7 @@ static void unqueue_me_pi(struct futex_q *q)
> __releases(q->lock_ptr)
> {
> WARN_ON(plist_node_empty(&q->list));
> - plist_del(&q->list,&q->list.plist);
> + __unqueue_futex(q);

and here

>
> BUG_ON(!q->pi_state);
> free_pi_state(q->pi_state);
> @@ -2133,7 +2150,7 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
> * We were woken prior to requeue by a timeout or a signal.
> * Unqueue the futex_q and determine which it was.
> */
> - plist_del(&q->list,&q->list.plist);
> + plist_del(&q->list,&hb->chain);
>
> /* Handle spurious wakeups gracefully */
> ret = -EWOULDBLOCK;

Thanks Lai!

--
Darren Hart
Yocto Linux Kernel


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