* * * * *
They never learn …
According to SlashDot, [1] there's a new OS (Operating System) called,
appropriately enough, NewOS. [2] Okay, I have an interest in these things,
and, unlike some others, [3] this one seems to actually be in a working
state.
I download the code and peruse it.
Not five minutes and already I find a horrible bug:
>
> void aquire_spinlock(int *lock)
> {
> if(smp_num_cpus > 1) {
> while(1) {
> while(*lock != 0)
> ;
> if(test_and_set(lock, 1) == 0)
> break;
> }
> }
> }
>
> void release_spinlock(int *lock)
> {
> *lock = 0;
> }
>
>
“newos/kernel/smp.c:48”
It works if you have more than one CPU (Central Processing Unit), but on your
typical single-CPU system, this fails to do The Right Thing and you end up
with very hard to track down bugs. Also, the code for test_and_set() is
needlessly complicated:
>
> test_and_set:
> movl 4(%esp),%edx /* load address of variable into edx */
>
> _test_and_set1:
> movl 8(%esp),%ecx /* load the value to set the lock to */
> movl (%edx),%eax /* load the value of variable into eax */
>
> cmpl $0, %eax /* see if someone else got it */
> jnz _test_and_set2 /* if so, bail out */
>
> lock
> cmpxchg %ecx,(%edx)
>
> jnz _test_and_set1 /* if zf = 0, cmpxchng failed so redo it */
>
> _test_and_set2:
> ret
>
>
“newos/kernel/arch/i386/arch_i386.S:61”
Sigh. It's not hard or complicated. [4]
[1]
http://slashdot.org/developers/01/05/23/0118250.shtml
[2]
http://www.newos.org/
[3]
http://www.v2.nl/v2_os/
[4]
gopher://gopher.conman.org/0Phlog:1999/12/08.2
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