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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Oct 24, 2016 at 02:18:17PM +0200, Juergen Gross wrote:
> On 24/10/16 14:06, Andrew Cooper wrote:
> > 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@xxxxxxxx>
> >>> 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.
> 
> Okay, so either oxenstored should be corrected by some ocaml capable
> developer, or I can send a patch which will limit the bug statement to
> oxenstored.
> 

A patch to spell out limitation on oxenstored works for me.

Wei.

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