Re: [PATCH V4 1/1] rcu: introduce kfree_rcu()

Paul E. McKenney
2011-03-16T04:08:17Z

On Tue, Mar 15, 2011 at 02:07:24PM +0100, Arnd Bergmann wrote:
> On Tuesday 15 March 2011, Paul E. McKenney wrote:
> > And it makes use of statically allocated structures a bit clunky.
>
> How do statically allocated structures relate to this? I would
> expect that you never call kfree_rcu on them, so it shouldn't
> matter.
>
> > Yet another approach is to use the low-order bit of the rcu_head pointer,
> > given that the rcu_head structure does have to be aligned. If this bit
> > is set, then the function pointer could be interpreted as an offset.
> > This approach might also allow a slab_free_rcu() to be constructed, given
> > that the full 32 bits of the function pointer would be available.
> > For example, if the upper 16 bits are zero, the low-order 16 bits are
> > the offset. If the upper 16 bits are 0x1, then the low-order 16 bits
> > might be an index that selects the desired slab cache.
>
> This solution sounds like a clear improvement over the patch that Lai
> Jiangshan posted, without any downsides.

Except that I was forgetting that we don't really have any way to stop
people from handing us misaligned rcu_head structures -- that topic came
up last time as well. Or were the people mentioning that possibility
being overly paranoid?

Thanx, Paul