[34-longterm 195/247] call_function_many: fix list delete vs add race

Paul Gortmaker
2011-06-23T17:38:37Z

From: Milton Miller <miltonm@bga.com>

-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
If you see a problem with using this for longterm, please comment.
-------------------

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
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.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
kernel/smp.c | 20 +++++++++++++-------
1 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 0a9e6bd..de11a66 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -470,14 +470,15 @@ void smp_call_function_many(const struct cpumask *mask,
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);
/*
@@ -486,6 +487,11 @@ void smp_call_function_many(const struct cpumask *mask,
* 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);

/*
--
1.7.4.4