Partition problem

Tim Cussins timcussins at eml.cc
Tue Oct 23 08:12:27 CDT 2007


> 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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