[32/44] call_function_many: fix list delete vs add race
From: Greg KH
Date: Mon Mar 21 2011 - 19:57:18 EST
2.6.32-longterm review patch. If anyone has any objections, please let us know.
From: Milton Miller <miltonm@xxxxxxx>
commit e6cd1e07a185d5f9b0aa75e020df02d3c1c44940 upstream.
Peter pointed out there was nothing preventing the list_del_rcu in
smp_call_function_interrupt from running before the list_add_rcu in
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
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.
Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Milton Miller <miltonm@xxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
kernel/smp.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)
@@ -429,14 +429,15 @@ void smp_call_function_many(const struct
- * 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.
- atomic_set(&data->refs, cpumask_weight(data->cpumask));
@@ -445,6 +446,11 @@ void smp_call_function_many(const struct
* will not miss any other list entries:
+ * 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));
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