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Subject[PATCH 1/4 v3] call_function_many: fix list delete vs add race
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 <>
Signed-off-by: Milton Miller <>
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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