[PATCH 2/2] locking/qspinlock: Ensure node->count is updated before initialising node

[Will Deacon]

When queuing on the qspinlock, the count field for the current CPU's head
node is incremented. This needn't be atomic because locking in e.g. IRQ
context is balanced and so an IRQ will return with node->count as it
found it.

However, the compiler could in theory reorder the initialisation of
node[idx] before the increment of the head node->count, causing an
IRQ to overwrite the initialised node and potentially corrupt the lock
state.

Avoid the potential for this harmful compiler reordering by placing a
barrier() between the increment of the head node->count and the subsequent
node initialisation.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 kernel/locking/qspinlock.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 348c8cec1042..d880296245c5 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -379,6 +379,14 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	tail = encode_tail(smp_processor_id(), idx);
 
 	node += idx;
+
+	/*
+	 * Ensure that we increment the head node->count before initialising
+	 * the actual node. If the compiler is kind enough to reorder these
+	 * stores, then an IRQ could overwrite our assignments.
+	 */
+	barrier();
+
 	node->locked = 0;
 	node->next = NULL;
 	pv_init_node(node);
-- 
2.1.4