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

Grant Grundler (
Tue, 02 Jan 2001 12:39:46 -0800

Sorry for being dense - but I don't see the problem in using
a spinlock to implement xchg(). The example algorithm looks broken.
Or am I missing something obvious here?

"David S. Miller" wrote:
> 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);
> }

producer() looks broken. The problem is two producers can race and
one will put the wrong value of list->head in elem->next.

I think prepending to list->head needs to either be protected by a spinlock
or be a per-cpu data structure. consumer() should be ok assuming the code
can tolerate picking up "late arrivals" in the next pass.
Or am I missing something obvious here?

It's worse if producer were inlined: the arch specific optimisers might
re-order the "elem->next = list->head" statement to be quite a bit more
than 1 or 2 cycles from the xchg() operation.


Grant Grundler
Unix Systems Enablement Lab
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