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-rw-r--r--kernel/smp.c46
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);