[PATCH v2 5/7] connector/cn_proc: Protect send_msg() with a local lock

[Sebastian Andrzej Siewior]

From: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>

send_msg() disables preemption to avoid out-of-order messages. As the
code inside the preempt disabled section acquires regular spinlocks,
which are converted to 'sleeping' spinlocks on a PREEMPT_RT kernel and
eventually calls into a memory allocator, this conflicts with the RT
semantics.

Convert it to a local_lock which allows RT kernels to substitute them with
a real per CPU lock. On non RT kernels this maps to preempt_disable() as
before. No functional change.

[bigeasy: Patch description]

Cc: Evgeniy Polyakov <zbr@xxxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 drivers/connector/cn_proc.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/drivers/connector/cn_proc.c b/drivers/connector/cn_proc.c
index d58ce664da843..d424d1f469136 100644
--- a/drivers/connector/cn_proc.c
+++ b/drivers/connector/cn_proc.c
@@ -18,6 +18,7 @@
 #include <linux/pid_namespace.h>
 
 #include <linux/cn_proc.h>
+#include <linux/locallock.h>
 
 /*
  * Size of a cn_msg followed by a proc_event structure.  Since the
@@ -38,25 +39,32 @@ static inline struct cn_msg *buffer_to_cn_msg(__u8 *buffer)
 static atomic_t proc_event_num_listeners = ATOMIC_INIT(0);
 static struct cb_id cn_proc_event_id = { CN_IDX_PROC, CN_VAL_PROC };
 
-/* proc_event_counts is used as the sequence number of the netlink message */
-static DEFINE_PER_CPU(__u32, proc_event_counts) = { 0 };
+/* local_evt.counts is used as the sequence number of the netlink message */
+struct local_evt {
+	__u32 counts;
+	struct local_lock lock;
+};
+static DEFINE_PER_CPU(struct local_evt, local_evt) = {
+	.counts = 0,
+	.lock = INIT_LOCAL_LOCK(lock),
+};
 
 static inline void send_msg(struct cn_msg *msg)
 {
-	preempt_disable();
+	local_lock(&local_evt.lock);
 
-	msg->seq = __this_cpu_inc_return(proc_event_counts) - 1;
+	msg->seq = __this_cpu_inc_return(local_evt.counts) - 1;
 	((struct proc_event *)msg->data)->cpu = smp_processor_id();
 
 	/*
-	 * Preemption remains disabled during send to ensure the messages are
-	 * ordered according to their sequence numbers.
+	 * local_lock() disables preemption during send to ensure the messages
+	 * are ordered according to their sequence numbers.
 	 *
 	 * If cn_netlink_send() fails, the data is not sent.
 	 */
 	cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_NOWAIT);
 
-	preempt_enable();
+	local_unlock(&local_evt.lock);
 }
 
 void proc_fork_connector(struct task_struct *task)
-- 
2.27.0.rc0