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    Subject[PATCH 1/4 v3] call_function_many: fix list delete vs add race
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    Peter pointed out there was nothing preventing the list_del_rcu in
    smp_call_function_interrupt from running before the list_add_rcu in
    smp_call_function_many. Fix this by not setting refs until we
    have gotten the lock for the list. Take advantage of the wmb in
    list_add_rcu to save an explicit additional one.

    I tried to force this race with a udelay before the lock & list_add
    and by mixing all 64 online cpus with just 3 random cpus in the mask,
    but was unsuccessful. Still, inspection shows a valid race, and the
    fix is a extension of the existing protection window in the current code.

    Cc: stable (v2.6.32 and later)
    Reported-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Milton Miller <miltonm@bga.com>
    ---
    v2: rely on wmb in list_add_rcu not combined partial ordering of spin
    lock and unlock, which does not provide the needed guarantees.

    Index: linux-2.6/kernel/smp.c
    ===================================================================
    --- linux-2.6.orig/kernel/smp.c 2011-01-31 17:44:47.182756513 -0600
    +++ linux-2.6/kernel/smp.c 2011-01-31 18:25:47.266755387 -0600
    @@ -491,14 +491,15 @@ void smp_call_function_many(const struct
    cpumask_clear_cpu(this_cpu, data->cpumask);

    /*
    - * To ensure the interrupt handler gets an complete view
    - * we order the cpumask and refs writes and order the read
    - * of them in the interrupt handler. In addition we may
    - * only clear our own cpu bit from the mask.
    + * We reuse the call function data without waiting for any grace
    + * period after some other cpu removes it from the global queue.
    + * This means a cpu might find our data block as it is writen.
    + * The interrupt handler waits until it sees refs filled out
    + * while its cpu mask bit is set; here we may only clear our
    + * own cpu mask bit, and must wait to set refs until we are sure
    + * previous writes are complete and we have obtained the lock to
    + * add the element to the queue.
    */
    - smp_wmb();
    -
    - atomic_set(&data->refs, cpumask_weight(data->cpumask));

    raw_spin_lock_irqsave(&call_function.lock, flags);
    /*
    @@ -507,6 +508,11 @@ void smp_call_function_many(const struct
    * will not miss any other list entries:
    */
    list_add_rcu(&data->csd.list, &call_function.queue);
    + /*
    + * We rely on the wmb() in list_add_rcu to order the writes
    + * to func, data, and cpumask before this write to refs.
    + */
    + atomic_set(&data->refs, cpumask_weight(data->cpumask));
    raw_spin_unlock_irqrestore(&call_function.lock, flags);

    /*

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