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>The Players</H1
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> At any time each of the CPUs in a system can be:
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> not associated with any process, serving a hardware interrupt;
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> not associated with any process, serving a softirq, tasklet or bh;
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> running in kernel space, associated with a process;
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> running a process in user space.
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> There is a strict ordering between these: other than the last
category (userspace) each can only be pre-empted by those above.
For example, while a softirq is running on a CPU, no other
softirq will pre-empt it, but a hardware interrupt can. However,
any other CPUs in the system execute independently.
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> We'll see a number of ways that the user context can block
interrupts, to become truly non-preemptable.
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>User Context</H1
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> User context is when you are coming in from a system call or
other trap: you can sleep, and you own the CPU (except for
interrupts) until you call <TT
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>schedule()</TT
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In other words, user context (unlike userspace) is not pre-emptable.
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> You are always in user context on module load and unload,
and on operations on the block device layer.
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> In user context, the <TT
CLASS="VARNAME"
>current</TT
> pointer (indicating
the task we are currently executing) is valid, and
<TT
CLASS="FUNCTION"
>in_interrupt()</TT
>
(<TT
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>include/asm/hardirq.h</TT
>) is <SPAN
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</SPAN
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> Beware that if you have interrupts or bottom halves disabled
(see below), <TT
CLASS="FUNCTION"
>in_interrupt()</TT
> will return a
false positive.
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