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Subject[PATCH v4 4/5] net: qrtr: Make qrtr_port_lookup() use RCU
Date
The important part of qrtr_port_lookup() wrt synchronization is that the
function returns a reference counted struct qrtr_sock, or fail.

As such we need only to ensure that an decrement of the object's
refcount happens inbetween the finding of the object in the idr and
qrtr_port_lookup()'s own increment of the object.

By using RCU and putting a synchronization point after we remove the
mapping from the idr, but before it can be released we achieve this -
with the benefit of not having to hold the mutex in qrtr_port_lookup().

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
---

Changes since v3:
- None

net/qrtr/qrtr.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/qrtr/qrtr.c b/net/qrtr/qrtr.c
index 52816d44fb26..8ae301132a54 100644
--- a/net/qrtr/qrtr.c
+++ b/net/qrtr/qrtr.c
@@ -646,11 +646,11 @@ static struct qrtr_sock *qrtr_port_lookup(int port)
if (port == QRTR_PORT_CTRL)
port = 0;

- mutex_lock(&qrtr_port_lock);
+ rcu_read_lock();
ipc = idr_find(&qrtr_ports, port);
if (ipc)
sock_hold(&ipc->sk);
- mutex_unlock(&qrtr_port_lock);
+ rcu_read_unlock();

return ipc;
}
@@ -692,6 +692,10 @@ static void qrtr_port_remove(struct qrtr_sock *ipc)
mutex_lock(&qrtr_port_lock);
idr_remove(&qrtr_ports, port);
mutex_unlock(&qrtr_port_lock);
+
+ /* Ensure that if qrtr_port_lookup() did enter the RCU read section we
+ * wait for it to up increment the refcount */
+ synchronize_rcu();
}

/* Assign port number to socket.
--
2.24.0
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