Messages in this thread | | | Date | Wed, 11 Nov 2015 19:54:15 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction |
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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().
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