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Re: Atomic Operations (continuing on from Masking Interrupts?)
>>>>> "David" == David Querbach <email@example.com> writes:
>> The fix for this problem is to have a counter that increments
>> on disable, and decrements on enable. The enable routine will
>> only actually enable interrupts if the value of the counter is
>> zero. We must make sure that the increment and decrement
>> operations are also atomic; therefore, we must actually disable
>> interrupts before the counter is incremented in
>> assembler_routine_to_disable_interrupts(), and decrement before
>> we disable interrupts in
>> assembler_routine_to_enable_interrupts() (we realize, of
>> course, that it is an error to call
>> assembler_routine_to_enable_interrupts() when interrupts are
>> not disabled!), and only actually enable interrupts in
>> assembler_routine_to_enable_interrupts() is the counter goes to
>> zero. The counter must be global in C, but should probably be
>> class static in C++.
David> In our system, we have a target-dependent header file that
David> defines four macros:
David> - enableInts() unconditionally enables interrupts
David> - disableInts() unconditionally disables interrupts
David> - inhibitInts() saves the CPU's interrupt mask bit in an
David> auto variable, then invokes disableInts()
David> - restoreInts() sets the CPU's interrupt mask bit from
David> the auto variable written by inhibitInts().
David> This method isn't perfect, but it does allow inhibiting
David> interrupts deep in a library without interfering with the
David> application's interrupt enable/disable status. Also, since
David> we have such a file for each processor, interrupt mask
David> handling becomes portable.
The problem is that there is no language enforced way to insure that
enables are balanced with disables, and inhibits are balanced with
restores. The use of a subroutine that takes a thunk as its sole
argument, such as my "without_pre_emption(thunk)" example above,
forces the programmer to keep things balanced. The only real
trouble-maker is throwing an exception, or taking a longjump. Lisp
provides "unwind-protect" for that situation.
-------- "And there came a writing to him from Elijah" [2Ch 21:12] --------
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