Subject: Re: [linux-audio-dev] priority inversion & inheritance
From: Martijn Sipkema (msipkema_AT_sipkema-digital.com)
Date: Fri Jul 12 2002 - 01:15:25 EEST
> Let\'s say we have threads A (SCHED_FIFO/50) and B (SCHED_FIFO/90). If both
> threads are runnable, B will always be selected and it will run until
> (quoting the man page) \"... either it is blocked by an I/O request, it is
> preempted by a higher priority process, or it calls sched_yield\".
>
> But if B blocks on a mutex currently owned by A, B will then become
> blocked and A will be selected to run. A can now run until it unlocks
> the mutex, B becomes unblocked and once the next scheduling decision
> occurs, B will again be chosen to run.
>
> But it\'s true that if we have more threads the situation gets complex
> pretty quickly. Let\'s say we have C (SCHED_FIFO/70) that holds a resource
> needed by B, and is currently blocked by a long i/o operation. Then all
> three threads will be stuck for an unbounded time.
>
> Do correct me if this seems wrong to you.
This is correct. Even if the thread C is running in another application
and doesn\'t share any resources with A and B it will still be able to
preempt A (whilst holding the mutex) and thus block C indefinately. This
is called priority inversion. In any realistic environment there will
always be a thread C.
This is why I said the threads should be the same priority.
--martijn
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