Re: [PATCH] docs: remove wrong statement about bug in xenstore

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 24/10/16 12:49, Juergen Gross wrote:
> On 24/10/16 13:41, Wei Liu wrote:
>> On Mon, Oct 24, 2016 at 01:27:17PM +0200, Juergen Gross wrote:
>>> docs/misc/xenstore.txt states that xenstored will use "0" as a valid
>>> transaction id after 2^32 transactions. This is not true. Remove that
>>> statement.
>>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Can you point me to the relevant code snippet? Better still I would like
>> to see why it is the case in commit message.
> Sure: tools/xenstore/xenstored_transaction.c
>
> do_transaction_start():
> ...
>         /* Pick an unused transaction identifier. */
>         do {
>                 trans->id = conn->next_transaction_id;
>                 exists = transaction_lookup(conn,
> conn->next_transaction_id++);
>         } while (!IS_ERR(exists));
>
> It should be noted here that conn->next_transaction_id is initialized
> to be 0. So the error would occur for the first transaction, too.

Cxenstored isn't the only xenstored implementation, and I can't see
anything in the Ocaml version which mitigates this issue.  Furthermore,
because Ocaml's int is 31 bits or 63 bits, I suspect a 64bit oxenstored
will become unusable when the transaction id hits 4 billion and an a
truncation occurs when writing the id into the ring.  A 32bit oxenstored
only uses half the available transaction id space, and does wrap around
to 0.

~Andrew

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