Bluetooth: Host: Ensure conn_ready access is thread safe

[Yagoor]

• The bt_dev.le.conn_ready list is accessed by the tx_processor
  which runs in a workqueue, but bt_conn_data_ready can be called
  from different threads so we need to make sure that nothing will
  trigger a context switch while we are manipulating the list since
  sys_slist_*() functions are not thread safe.
• This only happens if call to bt_conn_data_ready is performed
  from a preemptive task which can happen depending on the
  application.

[Thalley]

get_conn_ready also manipulates the list by doing

	if (should_stop_tx(conn)) {
		/* Move reference off the list and into the `conn` variable. */
		__maybe_unused sys_snode_t *s = sys_slist_get(&bt_dev.le.conn_ready);

		__ASSERT_NO_MSG(s == node);
		(void)atomic_set(&conn->_conn_ready_lock, 0);

		/* Append connection to list if it still has data */
		if (conn->has_data(conn)) {
			LOG_DBG("appending %p to back of TX queue", conn);
			bt_conn_data_ready(conn);
		}

		return conn;
	}

since sys_slist_get pops the list. Should this also be guarded by a k_sched_lock?

[Yagoor]

That one is okay, because get_conn_ready is called on bt_conn_tx_processor which is executed in the system workqueue. Since the system workqueue is being executed at a cooperative priority, we don't need it here.

[Thalley]

we don't need it here.

for now :D

[kartben]

@Yagoor this is failing in main, please have a look:

west twister -p unit_testing/unit_testing -s bluetooth.host.conn.test_default --no-detailed-test-id


...
: && /usr/bin/gcc -m32 -gdwarf-4 -T /__w/zephyr/zephyr/subsys/testsuite/include/zephyr/ztest_unittest.ld -T /__w/zephyr/zephyr/tests/bluetooth/host/conn/mocks/mock-sections.ld CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/testsuite/ztest/src/ztest.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/testsuite/ztest/src/ztest_mock.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/testsuite/ztest/src/ztest_rules.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/testsuite/ztest/src/ztest_defaults.c.obj CMakeFiles/testbinary.dir/src/main.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/bluetooth/host/conn.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/logging/log_minimal.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/lib/net_buf/buf.c.obj CMakeFiles/testbinary.dir/__w/zephyr/zephyr/lib/net_buf/buf_simple.c.obj -o testbinary  mocks/libmocks.a  host_mocks/libhost_mocks.a && :
/usr/bin/ld: CMakeFiles/testbinary.dir/__w/zephyr/zephyr/subsys/bluetooth/host/conn.c.obj: in function `bt_conn_data_ready':
/__w/zephyr/zephyr/subsys/bluetooth/host/conn.c:865: undefined reference to `k_sched_lock'
/usr/bin/ld: /__w/zephyr/zephyr/subsys/bluetooth/host/conn.c:868: undefined reference to `k_sched_unlock'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.
...