Re: [PATCH] eliminte NMI entry/ exit code

[Linus Torvalds <torvalds@osdl.org>]

On Fri, 12 Aug 2005, George Anzinger wrote:
>
> The NMI entry and exit code fiddles with bits in the preempt count.  If 
> an NMI happens while some other code is doing the same, bits will be 
> lost.

Why?

Even if an NMI happens in the middle of a read-modify-write sequence that 
is critical, the NMI exit is supposed to undo whatever it was that the NMI 
entry did, so the preempt counters are "safe" wrt NMI: they may change, 
but they always change back by the time anybody cares.

This, btw, is something we depend on wrt _normal_ interrupts too. It's why 
people can read/modify/write preempt count without having to disable 
interrupts.

			Linus
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