Gedare Bloom gedare at
Fri Jun 26 13:50:12 UTC 2015

On Fri, Jun 26, 2015 at 9:42 AM, Sebastian Huber
<sebastian.huber at> wrote:
> On 26/06/15 15:28, Gedare Bloom wrote:
>> Since this thread got migrated to devel a bit prematurely, I'll
>> back-stop some of the details and how I understand the state of this
>> project.
>> Saurabh is working toward solving
>> (so yes there is a ticket already, and it  should be referenced by
>> patches accordingly). As a first step he has proposed a modified way
>> to step-down inherited priorities when multiple mutexes are acquired
>> simultaneously by one thread and contended by others, i.e. nested
>> mutex access. We wanted assurance the proposal is sound, so we had him
>> implement a model of the current thread synchronization of RTEMS
>> within a Java-based model-checking framework, called Java Path Finder,
>> or JPF. His model shows the existing error with the current
>> STRICT_ORDER option, and then does not show a problem using his
>> proposed method for solving nested mutex acquisition.
>> However, validating the proposed method apparently required adding a
>> global lock around the two linked lists that are used in Saurabh's
>> proposed solution. My intuition is that the global lock to protect
>> these lists is not a big problem for uniprocessor, and for SMP we
>> should prefer to find other solutions at any rate. The lists should
>> not be long, and we should be able to find their bounds. One list is
>> bounded by the number of mutexes a thread may hold, and the other by
>> the number of threads that may contend a specific mutex.
> Does this new approach address the problem here:
Perhaps Saurabh can comment and model this problem too, and then we
can see. As I understood the test, a low-priority task holding
semaphore A blocks a mid-priority task holding semaphore B (contending
A), and then a high-priority task blocks on semaphore B, elevating the
priority of mid, but not low => a priority inversion occurs.

If Saurabh's solution iterates the mutexes held by mid when elevating
its priority, then this inversion also should be fixed.


> --
> Sebastian Huber, embedded brains GmbH
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