Re: [Xen-devel] Cancelling asynchronous operations in libxl

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Euan Harris writes ("Re: Cancelling asynchronous operations in libxl"):
> We've discussed the semantics of cancellation a bit more off-list and
> have come to two conclusions:
> 
>   1.  [...]
> 
>       We should rename the proposed libxl_ao_cancel() to libxl_ao_abort().

Unless someone objects to this, I will do this in my next
rebase/resend.

(CCing a slightly wider set of people who may be interested in libxl
API semantics.)

>       This function will be defined as a best-effort way to kill an
>       asynchronous operation, and will give no guarantees about the
>       state of the affected domain afterwards.   We may add a true
>       libxl_ao_cancel() function later, with better guarantees about the
>       state of the domain afterwards.   libxl_ao_abort(), as defined here,
>       covers many of our requirements in Xapi.

My plan for implementing (eventually) libxl_ao_cancel is that
it works my like abort, except that operations can:

 * block and unblock cancellation during critical sections

 * declare an ao "committed", causing cancellation requests to all fail

 * divert cancellation requests to a special handler (which could
   start to try to undo the operation, for example)

...
>       The semantics of libxl_ao_cancel/_abort() are defined as best effort,
>       so it suffices to have just two return codes:
> 
>         0: The request to cancel/abort has been noted, and it may or may 
>            not happen.   To find out which, check the eventual return code
>            of the async operation.
> 
>         ERROR_NOTFOUND: the operation to be cancelled has already completed.

ERROR_NOTFOUND might also mean that the operation has not yet
started.  For example, the call to libxl_domain_create_new might be on
its way into libxl and be waiting for the libxl ctx lock.

Ian.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel