[Libguestfs] [PATCH libnbd v3] lib: Atomically update h->state when leaving the locked region.

[Eric Blake]

On 6/8/19 1:05 PM, Richard W.M. Jones wrote:
> Split h->state into:
> 
>  - h->public_state = the state on entry to the locked region
>    This is also the atomically, publicly visible state.
> 
>  - h->state = the real current state of the handle
> 
> When we leave the locked region we update h->public_state with
> h->state, so that from outside the lock the handle appears to move
> atomically from its previous state to the final state without going
> through any intermediate states.
> 
> Some calls to ‘get_state’ become calls to ‘get_next_state’ if the need

they need

> the real state.  Others which need to see the publicly visible state
> are changed to ‘get_public_state’.
> 
> All calls to ‘set_state’ become ‘set_next_state’ because that is the
> real state that gets updated.
> 
> The purpose of this patch is to make it easier to reason about the
> state in lockless code.
> ---


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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