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    Subject[75/87] call_function_many: fix list delete vs add race
    2.6.37-stable review patch.  If anyone has any objections, please let us know.


    From: Milton Miller <>

    commit e6cd1e07a185d5f9b0aa75e020df02d3c1c44940 upstream.

    Peter pointed out there was nothing preventing the list_del_rcu in
    smp_call_function_interrupt from running before the list_add_rcu in

    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

    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.

    Reported-by: Peter Zijlstra <>
    Signed-off-by: Milton Miller <>
    Signed-off-by: Linus Torvalds <>
    Signed-off-by: Greg Kroah-Hartman <>

    kernel/smp.c | 20 +++++++++++++-------
    1 file changed, 13 insertions(+), 7 deletions(-)

    --- a/kernel/smp.c
    +++ b/kernel/smp.c
    @@ -481,14 +481,15 @@ 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);
    @@ -497,6 +498,11 @@ 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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