Messages in this thread | | | Date | Tue, 2 Jan 2001 01:03:48 -0800 | From | "David S. Miller" <> | Subject | Re: [PATCH] move xchg/cmpxchg to atomic.h |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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