linux-kernel - [RT] remove_waiter does not need to do chain walk?(2.6.25.4-rt)

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Date:	Tue, 24 Jun 2008 16:07:58 +0800
From:	hyl <heyongli@...il.com>
To:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Cc:	mingo@...e.hu
Subject: [RT] remove_waiter does not need to do chain walk?(2.6.25.4-rt)

Hi, everyone

lets us just focus on remove_waiter in rt_spin_lock_slowlock
(2.6.25.4-rt).  refer to bellowing brief code .

i notice the comments above calling the  remove_waiter , but i can't
figure out the  sequence which meet
the comment.

I  do figure out a event sequence to proof  we must call
remove_waiter , but chain walk seems is not needed.

 0).  current process block on this lock (note:block on lock not the process)
 1).  adaptive_wait  continue  the loop without sleeping due to event
2): owner change( held no lock while adaptive wait)
 2.)  owner free the lock, another process be selected as pending
owner, then release lock
 2.x) then  current be boosted , and become pending owner's top
waiter, so pending owner be boosted too
 3.  in the new round loop: do_try_to_take_rt_mutex->try_to_steal_lock
lucky own the lock,
     and at this time, waiter.task is not NULL

 Question is: seems pending owner's block_on is null,   remove_waiter
seems need no chain walk?

 My scenario may not be the one of author,  please don't hesitate to
offer a example to clarity this question,
i think discuss about this make it clear and easy to maintain.


rt_spin_lock_slowlock(struct rt_mutex *lock)
{
      .........
	for (;;) {
		if (do_try_to_take_rt_mutex(lock, STEAL_LATERAL)) {
		...
		if (!waiter.task) {
		  . ..
		}
         	....
		if (adaptive_wait(&waiter, orig_owner)) {
		......
		}

	.....
	}
	....
	/*
	 * Extremely rare case, if we got woken up by a non-mutex wakeup,
	 * and we managed to steal the lock despite us not being the
	 * highest-prio waiter (due to SCHED_OTHER changing prio), then we
	 * can end up with a non-NULL waiter.task:
	 */
	if (unlikely(waiter.task))
		remove_waiter(lock, &waiter, flags);
.....
}

Regards
hyl
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