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

Paul E. McKenney
2011-03-16T04:38:28Z

On Wed, Mar 16, 2011 at 10:58:14AM +0800, Lai Jiangshan wrote:
> On 03/15/2011 09:07 PM, 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.
>
> This solution is good, but it changes too much code, I think we will switch to
> this solution until my posted solution can't work under some real bad situation
> happened.

Indeed, the bit patterns are totally internal to this patch, so we can
change as needed -- for example, if we later want to apply this same
technique to slab_free() as well as kfree().

Thanx, Paul