> Can someone give me a one-liner on what bottom-half functions are?
every device driver in unix (and operating systems modelled after
unix, such as linux) has two parts: the part which reads and writes
directly to the device, which runs in an interrupt context, and the
part which manipulates the i/o buffers, which runs in a process
context. the first part is called the bottom half or lower half; the
second part is called the top or upper half.
the reason for this separation is that bottom half functions have to
be as minimal as possible, because they run with interrupts disabled
and they can't block. also, since they don't run in a process context
they have no access to information about the process on whose behalf
the i/o is taking place. normally, a bottom half function merely
moves data between the device and an i/o buffer, and then immediately
awakens anything which is blocking on the buffer. when the process
which wants the buffer is scheduled in again, the top half functions
actually deal with the data.
--mirian
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