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

[Juergen Gross <jgross@xxxxxxxx>]

On 27.04.23 18:32, Julien Grall wrote:
> Hi,
>
> On 26/04/2023 08:08, Juergen Gross wrote:
> > 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):
> Hmmm... Not sure why I wrote "rewrite". I was meant to say that you want to 
> restore conn->in after acc_commit() is called.
> > You are suggesting to move setting conn->in to NULL to acc_commit() and
> > to restore it before returning? I'm fine with that.
> Either that or what I wrote above. It depends on whether you expect other caller 
> to be in the same situation.
I think it should be local to acc_commit(), as conn->in being NULL is a
requirement of the acc_commit() handling.

> > > >           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. :-)
> My concern is there is a disconnect between the enum and this macro. What would 
> happen if we increase the enum past ACC_REQ_N/ACC_TR_N?
This expansion is happening in later patches.

> Would it be necessary to update ACC_CHD_N?
With the #define above: no.

With including it in the enum: depends on the values of ACC_REQ_N and ACC_TR_N.
ACC_CHD_N must always be equal to the larger one of both.

In my current V6 variant I've added it to the enum with a comment:

+       ACC_CHD_N = ACC_TR_N,   /* max(ACC_REQ_N, ACC_TR_N), for changed dom. */


Juergen

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