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    Subject[36/44] smp_call_function_many: handle concurrent clearing of mask
    2.6.32-longterm review patch.  If anyone has any objections, please let us know.


    From: Milton Miller <>

    commit 723aae25d5cdb09962901d36d526b44d4be1051c upstream.

    Mike Galbraith reported finding a lockup ("perma-spin bug") where the
    cpumask passed to smp_call_function_many was cleared by other cpu(s)
    while a cpu was preparing its call_data block, resulting in no cpu to
    clear the last ref and unlock the block.

    Having cpus clear their bit asynchronously could be useful on a mask of
    cpus that might have a translation context, or cpus that need a push to
    complete an rcu window.

    Instead of adding a BUG_ON and requiring yet another cpumask copy, just
    detect the race and handle it.

    Note: arch_send_call_function_ipi_mask must still handle an empty
    cpumask because the data block is globally visible before the that arch
    callback is made. And (obviously) there are no guarantees to which cpus
    are notified if the mask is changed during the call; only cpus that were
    online and had their mask bit set during the whole call are guaranteed
    to be called.

    Reported-by: Mike Galbraith <>
    Reported-by: Jan Beulich <>
    Acked-by: Jan Beulich <>
    Signed-off-by: Milton Miller <>
    Signed-off-by: Linus Torvalds <>
    Signed-off-by: Greg Kroah-Hartman <>

    kernel/smp.c | 13 ++++++++++---
    1 file changed, 10 insertions(+), 3 deletions(-)

    --- a/kernel/smp.c
    +++ b/kernel/smp.c
    @@ -388,7 +388,7 @@ void smp_call_function_many(const struct
    struct call_function_data *data;
    unsigned long flags;
    - int cpu, next_cpu, this_cpu = smp_processor_id();
    + int refs, cpu, next_cpu, this_cpu = smp_processor_id();

    * Can deadlock when called with interrupts disabled.
    @@ -399,7 +399,7 @@ void smp_call_function_many(const struct
    WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
    && !oops_in_progress);

    - /* So, what's a CPU they want? Ignoring this one. */
    + /* Try to fastpath. So, what's a CPU they want? Ignoring this one. */
    cpu = cpumask_first_and(mask, cpu_online_mask);
    if (cpu == this_cpu)
    cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
    @@ -457,6 +457,13 @@ void smp_call_function_many(const struct
    /* We rely on the "and" being processed before the store */
    cpumask_and(data->cpumask, mask, cpu_online_mask);
    cpumask_clear_cpu(this_cpu, data->cpumask);
    + refs = cpumask_weight(data->cpumask);
    + /* Some callers race with other cpus changing the passed mask */
    + if (unlikely(!refs)) {
    + csd_unlock(&data->csd);
    + return;
    + }

    spin_lock_irqsave(&call_function.lock, flags);
    @@ -470,7 +477,7 @@ void smp_call_function_many(const struct
    * to the cpumask before this write to refs, which indicates
    * data is on the list and is ready to be processed.
    - atomic_set(&data->refs, cpumask_weight(data->cpumask));
    + atomic_set(&data->refs, refs);
    spin_unlock_irqrestore(&call_function.lock, flags);


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