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    SubjectRe: [PATCH 1/2] remove rcu_assign_pointer(NULL) penalty with type/macro safety
    On Wed, 13 Feb 2008 14:00:24 -0800
    "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

    > Hello!
    >
    > This is an updated version of the patch posted last November:
    >
    > http://archives.free.net.ph/message/20071201.003721.cd6ff17c.en.html
    >
    > This new version permits arguments with side effects, for example:
    >
    > rcu_assign_pointer(global_p, p++);
    >
    > and also verifies that the arguments are pointers, while still avoiding
    > the unnecessary memory barrier when assigning NULL to a pointer.
    > This memory-barrier avoidance means that rcu_assign_pointer() is now only
    > permitted for pointers (not array indexes), and so this version emits a
    > compiler warning if the first argument is not a pointer. I built a "make
    > allyesconfig" version on an x86 system, and received no such warnings.
    > If RCU is ever applied to array indexes, then the second patch in this
    > series should be applied, and the resulting rcu_assign_index() be used.
    >
    > Given the rather surprising history of subtlely broken implementations of
    > rcu_assign_pointer(), I took the precaution of generating a full set of
    > test cases and verified that memory barriers and compiler warnings were
    > emitted when required. I guess it is the simple things that get you...
    >
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > ---
    >
    > rcupdate.h | 16 ++++++++++++----
    > 1 file changed, 12 insertions(+), 4 deletions(-)
    >
    > diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h linux-2.6.24-rap/include/linux/rcupdate.h
    > --- linux-2.6.24/include/linux/rcupdate.h 2008-01-24 14:58:37.000000000 -0800
    > +++ linux-2.6.24-rap/include/linux/rcupdate.h 2008-02-13 13:36:47.000000000 -0800

    whoop, ancient kernel alert.

    > @@ -270,12 +270,20 @@ extern struct lockdep_map rcu_lock_map;
    > * structure after the pointer assignment. More importantly, this
    > * call documents which pointers will be dereferenced by RCU read-side
    > * code.
    > + *
    > + * Throws a compiler warning for non-pointer arguments.
    > + *
    > + * Does not insert a memory barrier for a NULL pointer.
    > */
    >
    > -#define rcu_assign_pointer(p, v) ({ \
    > - smp_wmb(); \
    > - (p) = (v); \
    > - })
    > +#define rcu_assign_pointer(p, v) \
    > + ({ \
    > + typeof(*p) *_________p1 = (v); \
    > + \
    > + if (!__builtin_constant_p(v) || (_________p1 != NULL)) \
    > + smp_wmb(); \
    > + (p) = _________p1; \
    > + })

    Someone already merged the dont-do-it-for-NULL patch so I reworked this
    appropriately. Was too lazy to update the changelog though.



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