On Mon, 4 Jun 2007, Alan Cox wrote:
> > The thing is, why *should* we care about comparing addresses? We'll give
>
> Because people use it to tell objects apart. All over the kernel we do
> things like
>
> if (inode1 == inode2)
But that only makes sense if your objects have meaning, which is not
possible with a zero-sized object.
Let's take an example: one of the few reasons to check for equality (or
inequality) is for locking order.
So on UP, the lock goes away, and let's say that you (insanely) only have
a spinlock in the structure in question, so you have a zero-sized
structure that you want to test ordering on because you want to avoid
ABBA deadlocks.
So you write your code as
double_lock()
{
if (ptr1 == ptr2) {
lock(ptr1->lock);
return;
}
if (ptr1 < ptr2) {
lock(ptr1->lock);
lock(ptr2->lock);
return;
}
lock(ptr2->lock);
lock(ptr1->lock);
}
and the interesting thing is that this actually *works* even for the case
where the lock has gone away: even though ptr1/ptr2 are always equal.
In other words, the only _valid_ reasons to compare pointers like this end
up degenerating into working cases even for a zero-sized pointer.
The exception is if you use the memory allocator as a "ID allocator", but
quite frankly, if you use a size of zero, it's your own damn problem.
Insane code is not an argument for insane behaviour.
If people can't be bothered to create a "random ID generator" themselves,
they had damn well better use "kmalloc(1)" rather than "kmalloc(0)" to get
a unique cookie. Asking the allocator to do something idiotic because some
idiot thinks a memory allocator is a cookie allocator is just crazy.
I can understand that things like user-level libraries have to take crazy
people into account, but the kernel internal libraries definitely do not.
(Right now we warn once for zero-sized allocations anyway, and all the
cases we've found so far are either bugs that would have been found with
ZERO_ALLOC_PTR or would have been perfectly fine with it, so I don't think
anybody really _is_ that insane in the kernel)
Linus
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