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    Subject[PATCH] fix misplaced mb() in rcu_enter/exit_nohz()
    Hello!

    In the process of writing up the mechanical proof of correctness for the
    dynticks/preemptable-RCU interface, I noticed misplaced memory barriers
    in rcu_enter_nohz() and rcu_exit_nohz(). This patch puts them in the
    right place and adds a comment. The key thing to keep in mind is that
    rcu_enter_nohz() is -exiting- the mode that can legally execute RCU
    read-side critical sections. The memory barrier must be between any
    potential RCU read-side critical sections and the increment of the per-CPU
    dynticks_progress_counter, and thus must come -before- this increment.
    And vice versa for rcu_exit_nohz().

    The locking in the scheduler is probably saving us for the moment.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---

    rcupreempt.h | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcupreempt.h linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h
    --- linux-2.6.25-rc6/include/linux/rcupreempt.h 2008-03-16 17:45:16.000000000 -0700
    +++ linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h 2008-03-16 17:59:24.000000000 -0700
    @@ -87,15 +87,15 @@ DECLARE_PER_CPU(long, dynticks_progress_

    static inline void rcu_enter_nohz(void)
    {
    + mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
    __get_cpu_var(dynticks_progress_counter)++;
    WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
    - mb();
    }

    static inline void rcu_exit_nohz(void)
    {
    - mb();
    __get_cpu_var(dynticks_progress_counter)++;
    + mb(); /* CPUs seeing ++ must see subsequent RCU read-side crit sects */
    WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
    }


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