Re: [patch] block layer: kmemcheck fixes

Ingo Molnar
2008-02-07T19:32:28Z

* 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.

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
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.

Ingo