locking/qspinlock: Ensure node->count is updated before initialising node

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.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1518528177-19169-3-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 348c8cec104266e245773e38a97d15c436e1a5e0..d880296245c5954c432c539e2f14f3ade1c4719b 100644 (file)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -379,6 +379,14 @@ queue:
        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);