diff options
| -rw-r--r-- | kernel/smp.c | 46 |
1 files changed, 33 insertions, 13 deletions
diff --git a/kernel/smp.c b/kernel/smp.c index aaeee20c5634..7c6ded5effd9 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -483,23 +483,42 @@ void smp_call_function_many(const struct cpumask *mask, data = &__get_cpu_var(cfd_data); csd_lock(&data->csd); - BUG_ON(atomic_read(&data->refs) || !cpumask_empty(data->cpumask)); - data->csd.func = func; - data->csd.info = info; - cpumask_and(data->cpumask, mask, cpu_online_mask); - cpumask_clear_cpu(this_cpu, data->cpumask); + /* This BUG_ON verifies our reuse assertions and can be removed */ + BUG_ON(atomic_read(&data->refs) || !cpumask_empty(data->cpumask)); /* + * The global call function queue list add and delete are protected + * by a lock, but the list is traversed without any lock, relying + * on the rcu list add and delete to allow safe concurrent traversal. * 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. + * This means a cpu might find our data block as it is being + * filled out. + * + * We hold off the interrupt handler on the other cpu by + * ordering our writes to the cpu mask vs our setting of the + * refs counter. We assert only the cpu owning the data block + * will set a bit in cpumask, and each bit will only be cleared + * by the subject cpu. Each cpu must first find its bit is + * set and then check that refs is set indicating the element is + * ready to be processed, otherwise it must skip the entry. + * + * On the previous iteration refs was set to 0 by another cpu. + * To avoid the use of transitivity, set the counter to 0 here + * so the wmb will pair with the rmb in the interrupt handler. */ + atomic_set(&data->refs, 0); /* convert 3rd to 1st party write */ + + data->csd.func = func; + data->csd.info = info; + + /* Ensure 0 refs is visible before mask. Also orders func and info */ + smp_wmb(); + + /* 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); raw_spin_lock_irqsave(&call_function.lock, flags); /* @@ -509,8 +528,9 @@ void smp_call_function_many(const struct cpumask *mask, */ 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. + * We rely on the wmb() in list_add_rcu to complete our writes + * 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)); raw_spin_unlock_irqrestore(&call_function.lock, flags); |