The top half (the handler registered with request_irq(9)) normally moves data between the device and a memory buffer, ensures that the device is in a sane state, and little else. While the top half of a handler is running, the IRQ is question is blocked; if it is a fast interrupt handler (i.e., it has SA_INTERRUPT set), all interrupts are disabled.
The bottom half does whatever remains to be done. Bottom halves run with interrupts enabled, although a locking mechanism ensures that only one bottom half will be running at a given time. Bottom halves are run by do_bottom_half(), which is called from schedule() and ret_from_sys_call().
init_bh() installs routine() as bottom half number nr. It operates by adding an entry to the bh_base table, and setting the appropriate bit of the bh_mask vector. Rather than specifying a number explicitly, one should add an entry to the anonymous enum in include/linux/interrupt.h.
remove_bh() removes bottom half number nr from the list of bottom halves. It removes the entry from bh_base and clears the appropriate bit of bh_mask.
mark_bh() requests that the kernel run the specified bottom half at the first available opportunity. This function is normally called from the top half of an interrupt handler. It operates by setting the appropriate bit of the bh_active vector.
disable_bh() disables bottom half number nr by clearing the appropriate bit of bh_mask. This function also increments bh_mask_count[nr], which is used to ensure that nested calls to disable_bh() must be matched by an equal number of calls to enable_bh().
enable_bh() enables a bottom half previously disabled by disable_bh(). This function decrements bh_mask_count[nr]. Then, if that value is zero, the specified bottom half is enabled by setting the appropriate bit of bh_mask.
include/asm*/softirq.h, include/linux/interrupt.h, kernel/softirq.c
"Kernel Korner" in issue 26 of The Linux Journal includes a discussion of split-half interrupts under Linux. An online copy of this article can be found at http://www.ssc.com/lj/issue26/interrupt.html.