[TUHS] raise of interrupt level necessary
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.
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