The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Wednesday, May 23, 2001

They never learn …

According to SlashDot, there's a new OS called, appropriately enough, NewOS. Okay, I have an interest in these things, and, unlike some others, 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, 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.

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