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Re: Is this a bug


Nick Garnett wrote:

TomChen <chenqy_79@163.com> writes:



Hi All,

Kernel package src/sched/mlqueue.cxx
in function Cyg_ThreadQueue_Implementation::enqueue(Cyg_Thread *thread) ,

When there is more than one thread in the queue and the priority is
neither higer than the head nor lower than the tail, we have to search
the queue to find the place. The code for this is:

Cyg_Thread *qtmp = get_tail();
while ( thread->priority > qtmp->priority)
   qtmp = qtmp->get_prev();
qtmp->append( thread );

I think this should be:

Cyg_Thread *qtmp = get_tail();
while ( thread->priority *<* qtmp->priority)
qtmp = qtmp->get_prev();
qtmp->append( thread );



I believe the original code it correct.


Remember that the priorities go from 0=highest to 31=lowest. So the
loop spins while the thread priorities are numerically greater than
the one we want to insert. Once we find one that is less than or equal
we break out and install the new thread just after it.



Consider such a scenario, we enqueue three threads with priority 3, 5, 4 respectively into an empty queue in the order as shown. According to the originall code, we will get the result:

------next-----> ------next-----> ------next-----> T3
T3 T5 T4
T4 <-----prev----- <-----prev----- <-----prev------
| |
head tail


Now, when we try get_tail() to get the tail who has the lower pirority then the thread with priority 4 will be returned.

In the queue the head has higher priority (numerically less) and the tail has lower priority (numerically greater). In the code, the search begins from the tail , here the condition thread->priority > qtmp->priority will never be true since before the loop we have checked

       if( thread->priority > get_tail()->priority )
       {
           // We are lower priority than any thread in the queue,
           // go in at the end.
           add_tail( thread );
       }
So, I think this is a bug.



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