Apologies for using examples not immediately relevant to Darwin,
but I don't want this idiom to become more prevalent. It is broken,
even if in not so obvious ways.
Doesn't the use of a compare-and-swap or similar pattern (such as
test-and-set) solve this problem without a lock at all?
void* instance(void)
{
if ( !OSCompareAndSwap( 0, -1, &sentinel))
{
/* initialize variables */
if ( ! myVariable )
myVariable = allocateObject();
/* etc */
sentinel = 1;
}
else {
while (sentinel != 1)
/* yield, whatever that function is...pthread_yield()? */;
}
return myVariable;
}
You could use myVariable as the sentinel also but that introduces other
dependencies, and besides you probably don't want to declare it
volatile. If you were to do that, to avoid the inline-ordering problem
you'd assign the output of allocateObject to a local variable, then to
the global.
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