[PATCH 4.9 15/70] locking/osq_lock: Fix osq_lock queue corruption

Greg Kroah-Hartman
2018-09-17T22:58:18Z

From: Prateek Sood <prsood@codeaurora.org>

commit 50972fe78f24f1cd0b9d7bbf1f87d2be9e4f412e upstream.

Fix ordering of link creation between node->prev and prev->next in
osq_lock(). A case in which the status of optimistic spin queue is
CPU6->CPU2 in which CPU6 has acquired the lock.

tail
v
,-. <- ,-.
|6| |2|
`-' -> `-'

At this point if CPU0 comes in to acquire osq_lock, it will update the
tail count.

CPU2 CPU0
----------------------------------

tail
v
,-. <- ,-. ,-.
|6| |2| |0|
`-' -> `-' `-'

After tail count update if CPU2 starts to unqueue itself from
optimistic spin queue, it will find an updated tail count with CPU0 and
update CPU2 node->next to NULL in osq_wait_next().

unqueue-A

tail
v
,-. <- ,-. ,-.
|6| |2| |0|
`-' `-' `-'

unqueue-B

->tail != curr && !node->next

If reordering of following stores happen then prev->next where prev
being CPU2 would be updated to point to CPU0 node:

tail
v
,-. <- ,-. ,-.
|6| |2| |0|
`-' `-' -> `-'

osq_wait_next()
node->next <- 0
xchg(node->next, NULL)

tail
v
,-. <- ,-. ,-.
|6| |2| |0|
`-' `-' `-'

unqueue-C

At this point if next instruction
WRITE_ONCE(next->prev, prev);
in CPU2 path is committed before the update of CPU0 node->prev = prev then
CPU0 node->prev will point to CPU6 node.

tail
v----------. v
,-. <- ,-. ,-.
|6| |2| |0|
`-' `-' `-'
`----------^

At this point if CPU0 path's node->prev = prev is committed resulting
in change of CPU0 prev back to CPU2 node. CPU2 node->next is NULL
currently,

tail
v
,-. <- ,-. <- ,-.
|6| |2| |0|
`-' `-' `-'
`----------^

so if CPU0 gets into unqueue path of osq_lock it will keep spinning
in infinite loop as condition prev->next == node will never be true.

Signed-off-by: Prateek Sood <prsood@codeaurora.org>
[ Added pictures, rewrote comments. ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: sramana@codeaurora.org
Link: http://lkml.kernel.org/r/1500040076-27626-1-git-send-email-prsood@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/locking/osq_lock.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -104,6 +104,19 @@ bool osq_lock(struct optimistic_spin_que

prev = decode_cpu(old);
node->prev = prev;
+
+ /*
+ * osq_lock() unqueue
+ *
+ * node->prev = prev osq_wait_next()
+ * WMB MB
+ * prev->next = node next->prev = prev // unqueue-C
+ *
+ * Here 'node->prev' and 'next->prev' are the same variable and we need
+ * to ensure these stores happen in-order to avoid corrupting the list.
+ */
+ smp_wmb();
+
WRITE_ONCE(prev->next, node);

/*