Re: p = kmalloc(sizeof(*p), )

Pekka J Enberg
2005-09-20T12:20:24Z

Hi Russell,

On Tue, 20 Sep 2005, Russell King wrote:
> Think about it some more. You've added a new member to struct foo.
> You want to fix up all the places which allocate struct foo to
> initialise this new member. Grepping for 'struct foo' returns 100
> files. Grepping for kmalloc in those 100 files returns 100 files.
>
> Do you open all 100 in an editor and manually try and locate the five
> kmalloc instances of this structure, and end up missing some.

Nope. I grep for assignments to other members of that struct.

On Tue, 20 Sep 2005, Russell King wrote:
> Or do you do the sane thing and use kmalloc(sizeof(struct foo), ...)
> and grep for "kmalloc[[:space:]]*(sizeof[[:space:]]*(struct foo)"
> which returns only the five files and fix those up with knowledge
> that you've found all the instances?

There are still statically allocated structs left. So neither heuristic
for figuring out initialization points is perfect.

On 9/18/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> Such shuffling around should be done in easy to review stages so that
> bugs can be found, and not a mega patch. This inherently means that
> for a structure name change, you don't end up with a new structure
> named the same as an old structure. And if you compile-test the
> stages, you find out if you missed the problem.

No disagreement here.

On 9/18/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> Your solution is better if the only thing you're concerned about is
> "are we allocating enough memory for this fixed size structure".
> It completely breaks if you are also concerned about "are we doing
> correct initialisation" or "are we allocating enough memory for this
> variable sized structure" both of which are far more important
> questions.
>
> *especially* when you consider that kmalloc is redzoned and therefore
> will catch the kinds of bugs you're suggesting.

Well, yes, but for initialization, I would prefer something like what Al
Viro suggested. To me, initialization is a separate issue from kmalloc. I
do get your point but I just don't think sizeof(struct foo) is the answer.

In all completeness, I would personally prefer the following form for
allocation and initialization which is readable, easy to get right, and
highly greppable:

struct foo *p = kmalloc(sizeof *p, ...);

*p = (struct foo) {
.bar = ...;
};

Unfortunately it doesn't seem like gcc is doing such a good job with it.

Pekka