On Thu, 13 Jun 1996 23:43:46 +0100 
 saw@sun0.urz.uni-heidelberg.de (Hauke Fath) wrote:


[ Scott sez...]
 > >Thanks to Jason Thorpe for pointing this out.  (HI JASON! :-)

sure ... Looking at the code, that panic just stunk of unwanted reentrance.

[ Hauke sez...]
 > YEA!
 > 
 > This helped  :-)
 > -- for the sbc driver, that is. ncrscsi kernels still don't run on my se/30.

Is it the _same_ panic with ncrscsi kernels?  If moving splimp -> spl4 
``fixes'' the bug for kernels with an sbc driver, it really ought to do 
the same thing for ncrscsi...

splimp is supposed to prevent reentrancy for routines that deal with 
memory allocation and management.  I.e. malloc() runs at splimp, parts of 
pmap run at splimp, etc.  This is a hysterical name left over from the 
days of Internet Message Processors ... If I remember how it worked 
correctly, you basically needed to allocate the mbuf for an incoming 
packet, and had to keep the IMP from interrupting while you did this 
(because, if it did, you'd have to allocate an mbuf ... there's the 
unwated reentrance), thus splimp.  This is why, in the early days of 
NetBSD, the network interface drivers had calls to splimp() (splnet was, 
in those days, used to block the network software interrupts).

NetBSD has since changed the definition of splnet to be defined as 
"blocks network hardware interrupts" and introduced splsoftnet, which is 
defined as "blocks network software interrupts".  splimp is, in its 
present state, poorly named :-)

So, to get back to my point, really splimp needs to block anything that 
could potentially do memory allocation ... or, in this case, frob 
(possibly in some non-obvious way) the pmap module...

ciao.

 -- save the ancient forests - http://www.bayarea.net/~thorpej/forest/ -- 
Jason R. Thorpe                                       thorpej@nas.nasa.gov
NASA Ames Research Center                               Home: 408.866.1912
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