output modules simplify locking for mixer reset and load

- fixes the deadlock in px4io ioctl mixer reset
   - px4io Run() locks (CDev semaphore)
   - mixer load goes through px4io ioctl MIXERIOCRESET which calls MixingOutput::resetMixerThreadSafe()
   - MixingOutput::resetMixerThreadSafe() stores a Command::Type::resetMixer command in an atomic variable, schedules a work queue cycle, then sleep spins until the command is cleared
   - the execution of the cycle eventually calls back into PX4IO::updateOutputs(), which tries to lock (and waits forever)

src/drivers/tap_esc/TAP_ESC.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (c) 2018-2021 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2018-2022 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -33,6 +33,8 @@
 
 #include "TAP_ESC.hpp"
 
+#include <px4_platform_common/sem.hpp>
+
 TAP_ESC::TAP_ESC(char const *const device, uint8_t channels_count):
 	CDev(TAP_ESC_DEVICE_PATH),
 	OutputModuleInterface(MODULE_NAME, px4::serial_port_to_wq(device)),
@@ -325,6 +327,8 @@ bool TAP_ESC::updateOutputs(bool stop_motors, uint16_t outputs[MAX_ACTUATORS], u
 
 void TAP_ESC::Run()
 {
+	SmartLock lock_guard(_lock);
+
 	if (should_exit()) {
 		ScheduleClear();
 		_mixing_output.unregister();
@@ -409,19 +413,19 @@ void TAP_ESC::Run()
 
 int TAP_ESC::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 {
-	int ret = OK;
+	SmartLock lock_guard(_lock);
 
-	lock();
+	int ret = OK;
 
 	switch (cmd) {
 	case MIXERIOCRESET:
-		_mixing_output.resetMixerThreadSafe();
+		_mixing_output.resetMixer();
 		break;
 
 	case MIXERIOCLOADBUF: {
 			const char *buf = (const char *)arg;
 			unsigned buflen = strlen(buf);
-			ret = _mixing_output.loadMixerThreadSafe(buf, buflen);
+			ret = _mixing_output.loadMixer(buf, buflen);
 			break;
 		}
 
@@ -431,8 +435,6 @@ int TAP_ESC::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 		break;
 	}
 
-	unlock();
-
 	return ret;
 }