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

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