On Thu, 10 May 2007, Li Yu wrote:
Hi, Steven.
Nice to meet you again.
I have read the rt-mutex-design.txt that you wrote. That is excellent
description of rt_mutex. But I have a question for rt_mutex.
As you said:
Now since mutexes can be defined by user-land applications, we don't
want a DOS
type of application that nests large amounts of mutexes to create a large
PI chain, and have the code holding spin locks while looking at a large
amount of data. So to prevent this, the implementation not only implements
a maximum lock depth, but also only holds at most two different locks at a
time, as it walks the PI chain. More about this below.
After read the implementation of rt_mutex_adjust_prio_chain(), I found
the we really require maximin lock depth (1024 default), but I can not
see the check for more same locks duplication. Does this doc is
inconsistent with code?
Thanks in advanced.
Good luck.
- Li Yu
At the label "again:" inside rt_mutex_adjust_prio_chain() no spinlocks are
held. That is the kernel can reschedule at that point in the loop. So if
you as a priority X task try to take a lock you will not delay any higher
priority than X by more than the amount of time it takes to go
around in the loop once. The max lock depth is just an extra safety.
The whole idea in a priority based real-time system is that the latency on
priorty X only depends on what is going on at priority X and higher
(including interrupt handlers). What ever goes on at lower priority can only
interfere with a fixed, predetermined, small amount of jitter.
Esben
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