wait queues

[ruben@mrbrklyn.com (Ruben Safir)]

I assume this is a different wait then the one we covered in call for
concurrency.


On 04/19/2015 09:23 PM, Ruben Safir wrote:
> I'm not pouring over Love's book in detail and the section in Chapter 4
> on the wait queue is implemented  in the text completely surprised me.
> 
> He is recommending that you have to right your own wait queue entry
> routine for every process?  Isn't that reckless?
> 
> He is suggesting
> 
> DEFINE_WAIT(wait) //what IS wait
> 
> 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 task 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 - create the wait queue
> 2 - adds &wait onto queue q
> 3 checks if condition is true, if not, enter a while loop
> 4 prepare_to_wait which changes the status of our &wait to
> TASK_INTERUPPABLE

see this here must mean that wait is something else?

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