Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction

From: Peter Zijlstra
Date: Wed Nov 11 2015 - 13:44:35 EST

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))

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