Re: [patch] block layer: kmemcheck fixes

Jens Axboe
2008-02-07T20:12:16Z

On Thu, Feb 07 2008, Ingo Molnar wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > On Thu, 7 Feb 2008, Ingo Molnar wrote:
> > > INIT_HLIST_NODE(&rq->hash);
> > > RB_CLEAR_NODE(&rq->rb_node);
> > > - rq->ioprio = 0;
> > > - rq->buffer = NULL;
> > > - rq->ref_count = 1;
> > > - rq->q = q;
> > > - rq->special = NULL;
> > > - rq->data_len = 0;
> > > - rq->data = NULL;
> > > - rq->nr_phys_segments = 0;
> > > - rq->sense = NULL;
> > > - rq->end_io = NULL;
> > > - rq->end_io_data = NULL;
> > > - rq->completion_data = NULL;
> > > - rq->next_rq = NULL;
> > > + rq->completion_data = NULL;
> > > + /* rq->elevator_private */
> > > + /* rq->elevator_private2 */
> > > + /* rq->rq_disk */
> > > + /* rq->start_time */
> > > + rq->nr_phys_segments = 0;
> > > + /* rq->nr_hw_segments */
> > > + rq->ioprio = 0;
> > > + rq->special = NULL;
> > > + rq->buffer = NULL;
> > ...
> >
> > Can we please just stop doing these one-by-one assignments, and just do
> > something like
> >
> > memset(rq, 0, sizeof(*rq));
> > rq->q = q;
> > rq->ref_count = 1;
> > INIT_HLIST_NODE(&rq->hash);
> > RB_CLEAR_NODE(&rq->rb_node);
> >
> > instead?
> >
> > The memset() is likely faster and smaller than one-by-one assignments
> > anyway, even if the one-by-ones can avoid initializing some field or
> > there ends up being a double initialization..
>
> i definitely agree and do that for all code i write.
>
> But if someone does item by item initialization for some crazy
> performance reason (networking folks tend to have such constructs), it
> should be done i think how i've done it in the patch: by systematically
> listing _every_ field in the structure, in the same order, and
> indicating it clearly when it is not initialized and why.

That assumes that people find the references in two places when adding
members to a structure, not very likely (people are lazy!).

> and there it already shows that we do not initialize a few other members
> that could cause problems later on:
>
> + rq->data_len = 0;
> + /* rq->sense_len */
> + rq->data = NULL;
> + rq->sense = NULL;
>
> why is sense_len not initialized - while data_len is? In any case, these

because sense isn't set, when someone sets ->sense they should set
sense_len as well.

> days the memclear instructions are dirt cheap and we should just always
> initialize everything to zero by default, especially if it's almost all
> zero-initialized anyway.

Completely agree, some of these are just dormant bugs waiting to happen.
Clearing everything is the sanest approach.

--
Jens Axboe