call_function_many: fix list delete vs add race

From: Milton Miller
Date: Fri Jan 28 2011 - 19:20:34 EST

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 put
the entry on the list. We can use the lock acquire and release instead
of a wmb.

Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Milton Miller <miltonm@xxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

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, it seems to be a valid race, and the fix
is a simple change to the current code.

Index: common/kernel/smp.c
--- common.orig/kernel/smp.c 2011-01-28 16:23:15.000000000 -0600
+++ common/kernel/smp.c 2011-01-28 16:55:01.000000000 -0600
@@ -491,15 +491,17 @@ 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. We use the acquire and release
+ * of the lock as a wmb() -- acquire prevents write moving up and
+ * release requires old writes are visible.
- smp_wmb();
- atomic_set(&data->refs, cpumask_weight(data->cpumask));
raw_spin_lock_irqsave(&call_function.lock, flags);
* Place entry at the _HEAD_ of the list, so that any cpu still
@@ -509,6 +511,8 @@ void smp_call_function_many(const struct
list_add_rcu(&data->csd.list, &call_function.queue);
raw_spin_unlock_irqrestore(&call_function.lock, flags);

+ atomic_set(&data->refs, cpumask_weight(data->cpumask));
* Make the list addition visible before sending the ipi.
* (IPIs must obey or appear to obey normal Linux cache
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