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

[ Upstream commit 11dc13224c975efcec96647a4768a6f1bb7a19a8 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index b2caec7315af5b622a00f8babfd55bb3f27fb315..a72f5df643f8d017706b5edb39bd90857f3d3a7a 100644 (file)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -495,6 +495,14 @@ 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);