linux-kernel - Re: [patch] block layer: kmemcheck fixes

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Date:	Thu, 7 Feb 2008 20:31:40 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	Alan.Brunelle@...com, arjan@...ux.intel.com, dgc@....com,
	npiggin@...e.de, Andrew Morton <akpm@...ux-foundation.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Pekka Enberg <penberg@...il.com>
Subject: Re: [patch] block layer: kmemcheck fixes


* Linus Torvalds <torvalds@...ux-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
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