Re: [Xen-devel] [PATCH 1/2] libxl/devd: fix a race with concurrent device addition/removal

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Roger Pau Monne writes ("[PATCH 1/2] libxl/devd: fix a race with concurrent 
device addition/removal"):
> Current code can free the libxl__device inside of the libxl__ddomain_device
> before the addition has finished if a removal happens while an addition is
> still in process:
> 
>   backend_watch_callback
>             |
>             v
>        add_device
>             |                 backend_watch_callback
>     (async operation)                   |
>             |                           v
>             |                     remove_device
>             |                           |
>             |                           V
>             |                    device_complete
>             |                 (free libxl__device)
>             v
>      device_complete
>   (deref libxl__device)
> 
> Fix this by creating a temporary copy of the libxl__device, that's
> tracked by the GC of the nested async operation. This ensures that
> the libxl__device used by the async operations cannot be freed while
> being used.

Doesn't this arrange that the remove hotplug script will be invoked
while the add hotplug script is still running ?  Is that really
desirable (or allowed!) ?

Ian.

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