Re: [PATCH for-4.15] tools/xenstored: liveupdate: Properly check long transaction

[Jürgen Groß <jgross@xxxxxxxx>]

On 03.03.21 18:05, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
>
> As XenStored is single-threaded, conn->ta_start_time will always be
> smaller than now. As we substract the latter from the former, it means
> a transaction will never be considered long running.
>
> Invert the two operands of the substraction in both lu_reject_reason()
> and lu_check_allowed(). In addition to that, the former also needs to
> check that conn->ta_start_time is not 0 (i.e the transaction is not
> active).
>
> Take the opportunity to document the return condition of
> lu_check_allowed().
>
> Fixes: e04e53a5be20 ("tools/xenstore: allow live update only with no transaction 
> active")
> Reported-by: Bjoern Doebel <doebel@xxxxxxxxx>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

> ---
>
> I am a bit puzzled on how -F is implemented. From my understanding we
> will force LiveUpdate when one of the following conditions is met:
>    1) All the active transactions are long running
>    2) If we didn't manage to LiveUpdate after N sec
>
> It is not quite clear why we need the both as 2) would indirectly cover
> 1). However 2) is probably unsafe as we may reset transactions for
> "well-behaving" guest.
>
> So I am thinking to send a patch to drop 2). Any opinions?
This will enable two guests working together to block LU by using
overlapping transactions:

Guest 1: ----- ----- -----  ...
Guest 2: -- ----- ----- --- ...

There is always a transaction active, but none is regarded to be
long running.


Juergen

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