Messages in this thread | | | Date | Tue, 24 Jun 2008 16:07:58 +0800 | From | hyl <> | Subject | [RT] remove_waiter does not need to do chain walk?(2.6.25.4-rt) |
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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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