On Wed, Nov 11, 2015 at 07:44:27PM +0100, Peter Zijlstra wrote:
On Wed, Nov 11, 2015 at 07:31:28PM +0100, Peter Zijlstra wrote:
Add new one that does 'fetch_and_add' ? What is the real use case it will be used for?
Look at all the atomic_{add,dec}_return*() users in the kernel. A typical example would be a reader-writer lock implementations. See include/asm-generic/rwsem.h for examples.
Maybe a better example would be refcounting, where you free on 0.
if (!fetch_add(&obj->ref, -1)) free(obj);
Urgh, too used to the atomic_add_return(), which returns post op. That wants to be:
if (fetch_add(&obj->ref, -1) == 1) free(obj);
Note that I would very much recommend _against_ encoding the post-op thing in instructions. It works for reversible operations (like add) but is pointless for irreversible operations (like or).
That is, given or_return(), you cannot reconstruct the state prior to the operation, so or_return() provides less information than fetch_or().