Priority inherance issue

xi yang hiyangxi at
Wed May 20 03:58:30 UTC 2009

2009/5/19 Aitor Viana <aitor.viana.sanchez at>:
> There is prob. a middle ground solution. To have less than 256 levels per
> semaphore hashed (hash table)
> regards,
> Aitor
> On Tue, May 19, 2009 at 11:37 AM, Manuel Coutinho
> <manuel.coutinho at> wrote:
>> Hi
>> I've already placed up for discussion, but it was thought that by
>> activating
>> strict order mutex would solve this.
>> Just tried to test in RTEMS 4.9.2 with strict order and the result is not
>> perfect.
>> So, the issue is that when using semaphore priority inherence protocol,
>> the
>> taskA priority (task which holds the semaphore) must be equal to the
>> highest
>> priority of the task blocked on any of the semaphores that taskA holds.
>> Currently, this does not happen in RTEMS.
Yeah, you are right. with STRICT_ORDER_MUTEX, when thread releases a
inherent mutex, the priority of the thread is decreased to the
priority before locking this inherent mutex.
Here is a problem in some situations, for example.
 Thread P with Priority 10 have got SemA,B,C.
 Then, thread Q with priority 1 acquires Sem A.
 Priority of Thread P will be improved to 1.
 At this time, if P wants to release Sem C,
 its priority will be decreased to 10 which is the priority before
locking Sem C.

Is this the problem what you have?
>> My guess to solve this issue, is that when a semaphore is released, the
>> new
>> priority of the calling thread (lets call it thread A) must be searched
>> through a list (or chain) of all the threads blocked on other semaphores
>> that thread A holds.
>> Of course, also, if a thread which is blocked on semaphore is deleted, it
>> has to be removed from that list.
I think there is a more simpler solution. For every inherent mutex, we
record two priorities. One is before_improved_priority which is
initialized to the priority when get this mutex. The other is
after_improved_priority which is initialized to the lowest priority.

When a thread T with priority P1 wants to acquire a inherent mutex, it
finds that priority improvement is needed and do it like this:
before_improved_priority = current holder's priority
after_improved_priority = P1

When the holder releases a inherent mutex
It needs to check

if(current_priority is not higher than before_improved_priority)
    return and do nothing

if(current_priority does not equal after_improved_priority)
    return and do nothing because the priority of holder is improved
by thread which waits for other mutex.

decrease the holder's priority to before_improved_priority

Then, we are able to make sure that after releases the mutex, the
priority of the holder is still equal to the highest priority of
threads who wait for mutex which the holder holds.

Welcome more comments.

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