Messages in this thread | | | Date | Mon, 23 Mar 2009 17:56:39 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH -mm 1/6] slab: introduce __kfree_rcu |
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* Christoph Lameter <cl@linux-foundation.org> wrote:
> On Mon, 23 Mar 2009, Ingo Molnar wrote: > > > > > * Christoph Lameter <cl@linux-foundation.org> wrote: > > > > > On Mon, 23 Mar 2009, Pekka Enberg wrote: > > > > > > > > +static inline void *portion_to_obj(void *portion) > > > > > +{ > > > > > + struct page *page = virt_to_head_page(portion); > > > > > + struct slab *slab = page_get_slab(page); > > > > > + struct kmem_cache *cache = page_get_cache(page); > > > > > + unsigned int offset = portion - slab->s_mem; > > > > > + unsigned int index = offset / cache->buffer_size; > > > > > + > > > > > + return index_to_obj(cache, slab, index); > > > > > +} > > > > > > > > A minor nit: I think this would be more readable if you separated > > > > variable declarations from the initializations. Also, you can probably > > > > drop the inline from the function declaration and let GCC decide what to > > > > do. > > > > > > Thats debatable. I find the setting up a number of variables that > > > are all dependend in the above manner very readable. They are > > > usually repetitive. Multiple functions use similar > > > initializations. > > > > I agree with Pekka, it's clearly more readable when separated out > > nicely: > > > > struct kmem_cache *cache; > > unsigned int offset; > > unsigned int index; > > struct page *page; > > struct slab *slab; > > > > page = virt_to_head_page(portion); > > slab = page_get_slab(page); > > cache = page_get_cache(page); > > > > offset = portion - slab->s_mem; > > index = offset / cache->buffer_size; > > > > The original form is hard to read due to lack of structure. > > Structure can also be established differently: > > static inline void *portion_to_obj(void *portion) > { > struct page *page = virt_to_head_page(portion); > struct slab *slab = page_get_slab(page); > struct kmem_cache *cache = page_get_cache(page); > > unsigned int offset = portion - slab->s_mem; > unsigned int index = offset / cache->buffer_size; > > return index_to_obj(cache, slab, index);
It's still not as readable to me as the version i posted above and confusing as well, due to the newline in the middle of local variable definitions.
> It would be good if the whole series of actions that need to be > taken in order for the function to "get to know" the slab the > object parms would be simpler. Like its done in ruby > > (page, slab, cache) = get_slab_info(portion) > > (offset, index) = get_position_info(slab, portion) > > But how can this be done in C without weird pointer passing?
The version i posted is pretty compact visually. The actual type enumeration is repetitive and it's often a meaningless pattern.
What matters is this sequence of symbols:
> > page = virt_to_head_page(portion); > > slab = page_get_slab(page); > > cache = page_get_cache(page); > > > > offset = portion - slab->s_mem; > > index = offset / cache->buffer_size;
... and anyone versed in slab internals will know the type of these variables without having to look them up. (using variable names consistently through a full subsystem is important for this reason)
Pairing them up with their base types just obscures the real logic.
That is one reason why i generally use the 'reverse christmas tree' type of local variable definition blocks:
> > struct kmem_cache *cache; > > unsigned int offset; > > unsigned int index; > > struct page *page; > > struct slab *slab;
As the trained eye will just want to skip over this as irrelevant fluff and the shape makes this the easiest (the less complex a shape is geometrically, the less 'eye skipping overhead' there is).
Anyway, these are nuances and if you go strictly by what's minimally required by Documentation/CodingStyle you can stop a lot sooner than having to bother about such fine details. The original version was certainly acceptable - it's just that IMO Pekka was right that it can be done better.
Ingo
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