Subject: Re: [linux-audio-dev] priority inversion & inheritance
From: Martijn Sipkema (msipkema_AT_sipkema-digital.com)
Date: Thu Jul 11 2002 - 01:27:13 EEST
> victor yodaiken wrote a nice article on why priority inheritance as a
> way of handling priority inversion isn\\\'t a particularly attractive
> solution.
I read this article, but I am not convinced. The only argument against
using priority inheritance is that it is supposed to have poor
performance. If that is true, then perhaps using priority ceiling is
better. For audio applications this would probably be easy, since they
are not that complex.
An examples situation for using priority inheritance
might be porting a read/write audio i/o application to a
callback based interface without too much effort. This can\\\'t in the
general case be done without adding latency, if there is no blocking
allowed in the callback function. But why not block? It takes time
, but it has a worst case bounded execution time. Yet only if there is
no priority inversion. And the only way to prevent that, without
knowing the priority of the callback function, is using priority
inheritance. (of course the read/write thread might want to be running
high priority SCHED_FIFO already in which case a normal mutex would
work also).
I think the article has some good examples of when not to use priority
inheritance, but there are still situation where using it is ok.
What do we have now to handle priority inversion in Linux? We haven\\\'t
got priority ceiling or priority inheritance. Should we set a high
priority just before acquiring a mutex that is used by a high priority
thread and lower the priority after it is released? Is this as efficient
as priority ceiling?
Is any design where a high and a low priority thread share access to some
resource flawed?
If the article is saying the programmer should be protected from missuse
of priority inheritance by not supplying it, isn\\\'t that like Pascal? (I
never liked Pascal).
--martijn
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