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    SubjectRe: [PATCH tip/core/urgent 1/4] rcu: add rcu_access_pointer and rcu_dereference_protected
    On Fri, Apr 09, 2010 at 09:57:35AM -0700, Paul E. McKenney wrote:
    > On Fri, Apr 09, 2010 at 11:24:18AM +0200, Eric Dumazet wrote:
    > > Le vendredi 09 avril 2010 à 10:10 +0100, David Howells a écrit :
    > > > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
    > > >
    > > > > +#define rcu_access_pointer(p) ((void *)ACCESS_ONCE(p))
    > > > > ...
    > > > > +#define rcu_access_pointer(p) ((void *)ACCESS_ONCE(p))
    > > >
    > > > There's no difference between your two versions of rcu_access_pointer(), so
    > > > you could move that whole construct outside of the #ifdef'ed section.
    > > >
    > > > Other than that:
    > > >
    > > > Acked-by: David Howells <dhowells@redhat.com>
    > >
    > > I also prefer to keep type checking if possible
    > >
    > > #define rcu_access_pointer(p) (ACCESS_ONCE(p))
    >
    > Yikes! I was obsessing on preventing people from dereferencing the
    > pointer, but you are quite right, they might be comparing it to a
    > constant pointer to check for array bounds or for use of a statically
    > allocated emergency-pool item.

    If you want to prevent people from deferencing the pointer directly, or
    from assigning it to some other pointer, you could use Sparse's
    __attribute__((noderef)) and
    __attribute__((address_space(...)).

    See the definitions of __user and __iomem in include/linux/compiler.h
    for instance. (And, looking at that file, now __percpu. Nice!)

    You'd then have to explicitly use __force at the point where you
    legitimately dereference it.

    - Josh Triplett
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