Partition problem

Joel Sherrill joel.sherrill at oarcorp.com
Tue Oct 23 09:06:50 CDT 2007


Leon Pollak wrote:
> Thank you, Tim.
> The issue you write is clear for me.
> But reading the docs, I was sure that partition is thread safe. Also, looking 
> into the code shows a lot of enable/disable thread dispatch.
> Is not this enough?
>
>   
partitions are supposed to be thread safe.  They are based upon
supercore chains and the partition get and return use chain
operations which disable interrupts during get/append operations.

It should be very safe.

Out of curiosity, if you do an objdump on your executable,
do you see anything being moved (by optimization)
outside interrupt or dispatching critical sections?

--joel
> Thanks again.
>
>
> On Tuesday 23 October 2007 15:12, Tim Cussins wrote:
>   
>>> I do not believe that someone will look into the following description of
>>> theproblem, but may be this is in the area of known problems in 4.7 and I
>>> simply waste my time...:-)
>>>
>>>
>>>
>>>
>>> I have two tasks working on the same partition - taking buffers and
>>> returning
>>> back. Buffers are all of the same size.
>>>
>>> When the tasks are working sequentially, everything is OK.
>>> But when I make them to work in parallel (one may interrupt the other),
>>> very
>>> quickly I receive the exception on rtems_partition_get_buffer, which
>>> shows me
>>> (after diving into the rtems code) that the element
>>> the_partition->Memory.first has garbage value (all the rest of the
>>> structure
>>> looks normal).
>>>       
>> Sounds to me like you need to protect your accesses to the partition
>> with a mutex...
>>
>> You may have run across one of the fundamental issues with
>> multi-threading - two threads that need to share a resource (ie your
>> "the_partition" structure) - if one thread is in the middle of modifying
>> the structure and is interrupted, the stucture could easily be in an
>> invalid (garbage) state when the other thread looks at it.
>>
>> You'll probably need to add code to each thread that acquires a mutex
>> (shared between the threads, ie "the_partition_mutex") before operating
>> on "the_partition" using the rtems_partition_get_buffer(). Of course it
>> will have to release the mutex after it is done.
>>
>> In this way access to the_partition is serialised (only one thread can
>> be modifying it at any time). Have a look on google/wikipedia for info
>> on mutexes and critical sections.
>>
>>     
>>> I tried to catch this with my bdm debugger, but after one full day I gave
>>> up -
>>> too complex to catch this at the moment of corruption as this value
>>> changes
>>> all the time.
>>>       
>> yeah, good luck trying to catch that one! :P
>>     
>
>   




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