Re: [PATCH] move xchg/cmpxchg to atomic.h

["David S. Miller" <davem@redhat.com>]

   Date: 	Tue, 2 Jan 2001 00:11:57 -0800 (PST)
   From: Grant Grundler <grundler@cup.hp.com>

   Fundemental problem is parisc only supports one atomic operation
   (LDCW/LDCD) and uses spinlocks for all atomic operations including
   xchg/cmpxchg.

Using spinlocks for the implementation of xchg on SMP might be
problematic.

If you implement things like this, several subtle things might
break.  For example, there is code in a few spots (or, at least at one
time there was) which assumed the update of the datum itself is atomic
and uses this assumption to do lock-free read-only accesses of the
data.

If you require an external agent (f.e. your spinlock) because you
cannot implement xchg with a real atomic sequence, this breaks the
above assumptions.

It is very common to do things like:

producer(elem)
{
	elem->next = list->head;
	xchg(&list->head, elem);
}

consumer()
{
	local_list = xchg(&list->head, NULL);
	for_each(elem, local_list)
		do_something(elem);
}

In fact we had code excatly like this in the buffer cache at one
point in time.

Later,
David S. Miller
davem@redhat.com
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