Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Priority inheritance
L
(5)
S
(20)
H
(12)
Three threads at different priorities.
Currently, H is running. S, a higher-priority server thread, doesn’t have anything to do
right now so it’s waiting for a message and is blocked in MsgReceive(). L would like
to run but is at a lower priority than H, which is running. Everything is as you’d
expect, right?
Now H has decided that it would like to go to sleep for 100 milliseconds — perhaps it
needs to wait for some slow hardware. At this point, L is running.
This is where things get interesting.
As part of its normal operation, L sends a message to the server thread S, causing S to
go READY and (because it’s the highest-priority thread that’s READY) to start running.
Unfortunately, the message that L sent to S was “Compute pi to 5000 decimal places.”
Obviously, this takes more than 100 milliseconds. Therefore, when H’s 100
milliseconds are up and H goes READY, guess what? It won’t run, because S is
READY and at a higher priority!
What happened is that a low-priority thread prevented a higher-priority thread from
running by leveraging the CPU via an even higher-priority thread. This is priority
inversion.
To fix it, we need to talk about priority inheritance. A simple fix is to have the server,
S, inherit the priority of the client thread:
SEND-blocked
L
(5)
S
(5)
H
(12)
READY
REPLY-blocked
Blocked threads.
April 30, 2009 Chapter 2 • Message Passing 131