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>Hard IRQ Context</H1
><P
> Hardware interrupts usually communicate with a bottom half,
tasklet or softirq. Frequently this involves putting work in a
queue, which the BH/softirq will take out.
</P
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>Locking Between Hard IRQ and Softirqs/Tasklets/BHs</H1
><P
> If a hardware irq handler shares data with a softirq, you have
two concerns. Firstly, the softirq processing can be
interrupted by a hardware interrupt, and secondly, the
critical region could be entered by a hardware interrupt on
another CPU. This is where <TT
CLASS="FUNCTION"
>spin_lock_irq()</TT
> is
used. It is defined to disable interrupts on that cpu, then grab
the lock. <TT
CLASS="FUNCTION"
>spin_unlock_irq()</TT
> does the reverse.
</P
><P
> This works perfectly for UP as well: the spin lock vanishes,
and this macro simply becomes <TT
CLASS="FUNCTION"
>local_irq_disable()</TT
>
(<TT
CLASS="FILENAME"
>include/asm/smp.h</TT
>), which
protects you from the softirq/tasklet/BH being run.
</P
><P
> <TT
CLASS="FUNCTION"
>spin_lock_irqsave()</TT
>
(<TT
CLASS="FILENAME"
>include/linux/spinlock.h</TT
>) is a variant
which saves whether interrupts were on or off in a flags word,
which is passed to <TT
CLASS="FUNCTION"
>spin_lock_irqrestore()</TT
>. This
means that the same code can be used inside an hard irq handler (where
interrupts are already off) and in softirqs (where the irq
disabling is required).
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