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    Subject[PATCH 1/3] RCU: rcu_assign_pointer() removal of memory barriers
    This patch adds the rcu_assign_pointer() API that helps reduce the
    need for explicit memory barriers in code that uses RCU. This API
    buries the required memory barriers in a macro that also does the
    assignment. This has been tested successfully on i386 and ppc64.

    Signed-off-by: <paulmck@us.ibm.com>

    ---

    rcupdate.h | 18 ++++++++++++++++++
    1 files changed, 18 insertions(+)

    diff -urpN -X ../dontdiff linux-2.5/include/linux/rcupdate.h linux-2.5-rap/include/linux/rcupdate.h
    --- linux-2.5/include/linux/rcupdate.h Tue Sep 7 10:04:29 2004
    +++ linux-2.5-rap/include/linux/rcupdate.h Tue Sep 7 12:12:09 2004
    @@ -238,6 +238,24 @@ static inline int rcu_pending(int cpu)
    (_________p1); \
    })

    +/**
    + * rcu_assign_pointer - assign (publicize) a pointer to a newly
    + * initialized structure that will be dereferenced by RCU read-side
    + * critical sections. Returns the value assigned.
    + *
    + * Inserts memory barriers on architectures that require them
    + * (pretty much all of them other than x86), and also prevents
    + * the compiler from reordering the code that initializes the
    + * structure after the pointer assignment. More importantly, this
    + * call documents which pointers will be dereferenced by RCU read-side
    + * code.
    + */
    +
    +#define rcu_assign_pointer(p, v) ({ \
    + smp_wmb(); \
    + (p) = (v); \
    + })
    +
    extern void rcu_init(void);
    extern void rcu_check_callbacks(int cpu, int user);
    extern void rcu_restart_cpu(int cpu);
    -
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