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List:       tuhs
Subject:    [TUHS] raise of interrupt level necessary
From:       helbig () Informatik ! BA-Stuttgart ! DE (Wolfgang Helbig)
Date:       2002-12-07 22:33:23
Message-ID: 200212080035.gB80Z7t00512 () bsd ! korb
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Hi,

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.

Greetings,

Wolfgang Helbig




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