Re: [PATCH v4 04/13] tools/xenstore: add framework to commit accounting data on success only

[Juergen Gross <jgross@xxxxxxxx>]

On 25.04.23 19:52, Julien Grall wrote:
> Hi Juergen,
>
> On 05/04/2023 08:03, Juergen Gross wrote:
> > Instead of modifying accounting data and undo those modifications in
> > case of an error during further processing, add a framework for
> > collecting the needed changes and commit them only when the whole
> > operation has succeeded.
> >
> > This scheme can reuse large parts of the per transaction accounting.
> > The changed_domain handling can be reused, but the array size of the
> > accounting data should be possible to be different for both use cases.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> > V3:
> > - call acc_commit() earlier (Julien Grall)
> > - add assert() to acc_commit()
> > - use fixed sized acc array in struct changed_domain (Julien Grall)
> > ---
> >   tools/xenstore/xenstored_core.c   |  9 ++++--
> >   tools/xenstore/xenstored_core.h   |  3 ++
> >   tools/xenstore/xenstored_domain.c | 53 ++++++++++++++++++++++++++++++-
> >   tools/xenstore/xenstored_domain.h |  5 ++-
> >   4 files changed, 66 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
> > index 3ca68681e3..84335f5f3d 100644
> > --- a/tools/xenstore/xenstored_core.c
> > +++ b/tools/xenstore/xenstored_core.c
> > @@ -1023,6 +1023,9 @@ static void send_error(struct connection *conn, int error)
> >               break;
> >           }
> >       }
> > +
> > +    acc_drop(conn);
> > +
> >       send_reply(conn, XS_ERROR, xsd_errors[i].errstring,
> >                 strlen(xsd_errors[i].errstring) + 1);
> >   }
> > @@ -1034,6 +1037,9 @@ void send_reply(struct connection *conn, enum 
> > xsd_sockmsg_type type,
> >       assert(type != XS_WATCH_EVENT);
> > +    conn->in = NULL;
> AFAIU, you are setting conn->in to NULL in order to please..
>
> > +    acc_commit(conn);
> ... this call. However in case of an error like...
>
> > +
> >       if ( len > XENSTORE_PAYLOAD_MAX ) { >           send_error(conn, E2BIG);
> ... here, send_reply() will be called again. But the error will not be set 
> because conn->in is NULL.
> So I think you want to restore conn->in rewrite acc_commit(). The ordering would 
> also deserve an explanation in a comment.
Just to make sure I understand you correctly (I have some difficulties
parsing "So I think you want to restore conn->in rewrite acc_commit()."
completely):

You are suggesting to move setting conn->in to NULL to acc_commit() and
to restore it before returning? I'm fine with that.

> >           return;
> > @@ -1059,8 +1065,6 @@ void send_reply(struct connection *conn, enum 
> > xsd_sockmsg_type type,
> >           }
> >       }
> > -    conn->in = NULL;
> > -
> >       /* Update relevant header fields and fill in the message body. */
> >       bdata->hdr.msg.type = type;
> >       bdata->hdr.msg.len = len;
> > @@ -2195,6 +2199,7 @@ struct connection *new_connection(const struct 
> > interface_funcs *funcs)
> >       new->is_stalled = false;
> >       new->transaction_started = 0;
> >       INIT_LIST_HEAD(&new->out_list);
> > +    INIT_LIST_HEAD(&new->acc_list);
> >       INIT_LIST_HEAD(&new->ref_list);
> >       INIT_LIST_HEAD(&new->watches);
> >       INIT_LIST_HEAD(&new->transaction_list);
> > diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
> > index c59b06551f..1f811f38cb 100644
> > --- a/tools/xenstore/xenstored_core.h
> > +++ b/tools/xenstore/xenstored_core.h
> > @@ -139,6 +139,9 @@ struct connection
> >       struct list_head out_list;
> >       uint64_t timeout_msec;
> > +    /* Not yet committed accounting data (valid if in != NULL). */
> > +    struct list_head acc_list;
> > +
> >       /* Referenced requests no longer pending. */
> >       struct list_head ref_list;
> > diff --git a/tools/xenstore/xenstored_domain.c 
> > b/tools/xenstore/xenstored_domain.c
> > index 30fb9acec6..144cbafb73 100644
> > --- a/tools/xenstore/xenstored_domain.c
> > +++ b/tools/xenstore/xenstored_domain.c
> > @@ -91,6 +91,8 @@ struct domain
> >       bool wrl_delay_logged;
> >   };
> > +#define ACC_CHD_N (ACC_TR_N < ACC_REQ_N ? ACC_REQ_N : ACC_TR_N)
> Both ACC_TR_N and ACC_REQ_N are fixed. Can you explain why we need this magic?
>
> Related, wouldn't it be better to define it in the enum?
I can do that, of course. I just didn't want to make the enum even more
complex. :-)

But with a comment this should be okay IMO.


Juergen

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