[TUHS] raise of interrupt level necessary

Wolfgang Helbig helbig at Informatik.BA-Stuttgart.DE
Sun Dec 8 08:33:23 AEST 2002


While studying the V6 kernel, I search for some rules as when to raise
the interrupt priority level (IPL). I came up with something like this:
A kernel process needs to raise the IPL if a change of data must be atomic and might 
be accessed (not necessarily changed) by an IRS.

In light of this rule, some raising of interrupts seem unnecessary:
- In m40.s changes of the user space segmentation registers are bracketed by raising 
and lowering the IPL, although these registers are never accessed by an ISR.
- Likewise, in m40.s, just before the trap routine checks the runrun flag and 
restores r0, r1 and the user stackpointer, it raises the IPL. I can't see what 
possibly should go wrong, if being interrupted while restoring the values.
I'd appreciate any help from someone with more experience in kernel programming.


Wolfgang Helbig

More information about the TUHS mailing list