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    SubjectRe: [PATCH] Remove never-migrates assumption from rcu_process_callbacks()

    * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

    > Hello again!
    >
    > This patch fixes a potentially invalid access to a per-CPU variable in
    > rcu_process_callbacks(). This per-CPU access needs to be done in such
    > a way as to guarantee that the code using it cannot move to some other
    > CPU before all uses of the value accessed have completed. Even though
    > this code is currently only invoked from softirq context, which
    > currrently cannot migrate to some other CPU, life would be better if
    > this code did not silently make such an assumption.

    i've got the patch below queued up already - same thing, right?

    Ingo

    ----------->
    Subject: rcupreempt: fix hibernate/resume in presence of PREEMPT_RCU and hotplug
    From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Date: Wed, 27 Feb 2008 16:21:10 -0800

    This fixes a oops encountered when doing hibernate/resume in presence
    of PREEMPT_RCU. The problem was that the code failed to disable preemption
    when accessing a per-CPU variable. This is OK when called from code
    that already has preemption disabled, but such is not the case from
    the suspend/resume code path.

    Reported-by: Dave Young <hidave.darkstar@gmail.com>
    Tested-by: Dave Young <hidave.darkstar@gmail.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/rcupreempt.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    Index: linux/kernel/rcupreempt.c
    ===================================================================
    --- linux.orig/kernel/rcupreempt.c
    +++ linux/kernel/rcupreempt.c
    @@ -918,8 +918,9 @@ void rcu_offline_cpu(int cpu)
    * fix.
    */

    + local_irq_save(flags);
    rdp = RCU_DATA_ME();
    - spin_lock_irqsave(&rdp->lock, flags);
    + spin_lock(&rdp->lock);
    *rdp->nexttail = list;
    if (list)
    rdp->nexttail = tail;

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