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DATE 2015-04-01

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Key: id Value: 543110

Key: archive_learn Value: 2015-04-01

MESSAGE
DATE 2015-04-22
FROM Ruben Safir
SUBJECT Subject: [LIU Comp Sci] Fwd: Re: wait queues semiphores kernel implementations
From owner-learn-outgoing-at-mrbrklyn.com Wed Apr 22 13:28:15 2015
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Subject: [LIU Comp Sci] Fwd: Re: wait queues semiphores kernel implementations
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-------- Forwarded Message --------
Subject: Re: wait queues semiphores kernel implementations
Date: Wed, 22 Apr 2015 18:49:13 +0200
From: michi1-at-michaelblizek.twilightparadox.com
To: Ruben Safir
CC: kernelnewbies-at-kernelnewbies.org

Hi!

On 07:23 Wed 22 Apr , Ruben Safir wrote:
> Ruben QUOTED Previously:
>
> <<> Chapter 4 on the wait queue how it is implemented in the text
> completely surprised me.
>
> He is recommending that you have to write your own wait queue entry
> routine for every process? Isn't that reckless?
>
> He is suggesting
>
> DEFINE_WAIT(wait) //what IS wait EXACTLY in this context

#define DEFINE_WAIT_FUNC(name, function) \
wait_queue_t name = { \
.private = current, \
.func = function, \
.task_list = LIST_HEAD_INIT((name).task_list), \
}

#define DEFINE_WAIT(name) DEFINE_WAIT_FUNC(name, autoremove_wake_function)

> add_wait_queue(q, &wait); // in the current kernel this invovled
> // flag checking and a linked list
>
> while(!condition){ /* an event we are weighting for
> prepare_to_wait(&q, &wait, TASK_INTERRUPTIBLE);
> if(signal_pending(current))
> /* SIGNAl HANDLE */
> schedule();
> }
>
> finish_wait(&q, &wait);
>
> He also write how this proceeds to function and one part confuses me
>
> 5. When the taks awakens, it again checks whether the condition is
> true. If it is, it exists the loop. Otherwise it again calls schedule.
>
>
> This is not the order that it seems to follow according to the code.
>
> To me it looks like it should
> 1 - creat2 the wait queue
> 2 - adds &wait onto queue q
> 3 checks if condition is true, if so, if not, enter a while loop
> 4 prepare_to_wait which changes the status of our &wait to
> TASK_INTERUPPABLE
> 5 check for signals ... notice the process is still moving. Does it
> stop and wait now?
> 6 schedule itself on the runtime rbtree... which make NO sense unless
> there was a stopage I didn't know about.
> 7 check the condition again and repeat while look
> 7a. if the loop ends fishish_waiting... take it off the queue.

This is what wait_event_interruptable looks like:
http://lxr.linux.no/linux+*/include/linux/wait.h#L390

Seems like prepare_to_wait is now called before checking the condition and
add_wait_queue does not exist anymore.

> Isn't this reckless to leave this to users to write the code. Your
> begging for a race condition.

I agree. This is why I would not recommend it unless you have a good reason
to do so.

...
> Minus the Semiphore, that sounds like what we are doing with the wait
> list in the scheduler. But it looks like we are leaving it to the
> user. Why? It is similar but oddly different so I'm trying to figure
> out what is happening here.

The concept behind a waitqueue is more not about counting up+down. Basically
when you call wait_event_* you define what you are waiting for. For example
you have a socket and want to wait incoming data. Wheneven anything
happens to
the socket (e.g. data arrives, error, ...), somebody calls wake_up, your
thread makes up, check if the condition is true and then wait_event_* either
goes back to sleep or returns.

The difference is that you can have situations where wait_event_* returns
without anybody even having called wake_up. Also you can have situations
with
lots of calls to wake_up, but wait_event_* always goes back to sleep because
the events which happen do not cause your condition to become true.

-Michi
--
programing a layer 3+4 network protocol for mesh networks
see http://michaelblizek.twilightparadox.com




  1. 2015-04-01 Ruben <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] external paths
  2. 2015-04-01 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] omp pthread madness
  3. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Fw: Perl Developer / Linux SysAdmin Opportunities in South Florida
  4. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: omp pthread madness
  5. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: omp pthread madness
  6. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: [New discussion] which technologies/skills are good to have
  7. 2015-04-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: omp pthread madness
  8. 2015-04-05 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: Depths and heights
  9. 2015-04-09 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: Kernel thread scheduling
  10. 2015-04-09 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: Kernel thread scheduling
  11. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] BASh Shell Scripting
  12. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Chapter 5 Monitor
  13. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: [Perlweekly] #194 - The CPAN PR Challenge Now Has a Mini-Me!
  14. 2015-04-13 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Kernel Hacking Resources
  15. 2015-04-14 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Invitation: Vintage Computer Festival
  16. 2015-04-14 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] hashing slots sizes
  17. 2015-04-15 Ruben <mrbrklyn-at-panix.com> Re: [LIU Comp Sci] BASh Shell Scripting
  18. 2015-04-15 Maneesh Kongara <maneeshkongara-at-gmail.com> Re: [LIU Comp Sci] BASh Shell Scripting
  19. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Monday: Join 121 HackerNestologists at "HackerNest NYC April
  20. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: You're confirmed for: NYLUG Open hacker hours
  21. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Hash Tables Examples
  22. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: hashing quadradic probes
  23. 2015-04-15 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] [ruben-at-mrbrklyn.com: Re: Heap Management Problem]
  24. 2015-04-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Re: virtualbox
  25. 2015-04-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] linsched
  26. 2015-04-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] scheduler project thus far
  27. 2015-04-17 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Come Sunday Morning
  28. 2015-04-17 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Kernel Scheduling Project
  29. 2015-04-17 Ruben <ruben.safir-at-my.liu.edu> Subject: [LIU Comp Sci] more advanced coding musings ...
  30. 2015-04-19 Tony Genao <tony.genao-at-my.liu.edu> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  31. 2015-04-19 Ruben Safir <mrbrklyn-at-panix.com> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  32. 2015-04-19 Tony Genao <tony.genao-at-my.liu.edu> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  33. 2015-04-19 Ruben <mrbrklyn-at-panix.com> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  34. 2015-04-19 Tony Genao <tony.genao-at-my.liu.edu> Re: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  35. 2015-04-19 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: [NYLXS - HANGOUT] Summers Here
  36. 2015-04-19 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: The more I learn the less I know
  37. 2015-04-22 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Re: wait queues semiphores kernel implementations
  38. 2015-04-22 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] linux locks and mutex
  39. 2015-04-23 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Re: Kernel Scheduler and wiat queues
  40. 2015-04-25 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Kernel Scheduling and wait queues
  41. 2015-04-27 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: OS Cpt 9 27 question
  42. 2015-04-27 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Fwd: Tomorrow: You and 30 other Flatirons are going to "Flatiron
  43. 2015-04-28 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: DMCA exemption commenting process broken beyond repair
  44. 2015-04-29 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Do you have notes for this question
  45. 2015-04-29 Ruben Safir <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Professional Troll job available
  46. 2015-04-29 Ruben <mrbrklyn-at-panix.com> Subject: [LIU Comp Sci] Re: DMCA exemption commenting process broken beyond repair

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