Debug zio_wait() hangs (executor/waiter)

There exists a plausible cache concurrency issue with zio_wait().
This might occur because the zio->io_waiter to not assigned under
a lock in zio_wait(), is not checked under a lock in zio_done(),
and the zio may be dispatched to another thread for handling.
That said, none of the actual crash dumps I've looked at show
that this has ever occurred.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#2523

module/zfs/zio.c

@@ -1377,7 +1377,9 @@ __attribute__((always_inline))
 static inline void
 __zio_execute(zio_t *zio)
 {
+	mutex_enter(&zio->io_lock);
 	zio->io_executor = curthread;
+	mutex_exit(&zio->io_lock);
 
 	while (zio->io_stage < ZIO_STAGE_DONE) {
 		enum zio_stage pipeline = zio->io_pipeline;
@@ -1455,7 +1457,9 @@ zio_wait(zio_t *zio)
 	ASSERT(zio->io_stage == ZIO_STAGE_OPEN);
 	ASSERT(zio->io_executor == NULL);
 
+	mutex_enter(&zio->io_lock);
 	zio->io_waiter = curthread;
+	mutex_exit(&zio->io_lock);
 
 	__zio_execute(zio);
 
@@ -3316,12 +3320,13 @@ zio_done(zio_t *zio)
 		zio_notify_parent(pio, zio, ZIO_WAIT_DONE);
 	}
 
+	mutex_enter(&zio->io_lock);
 	if (zio->io_waiter != NULL) {
-		mutex_enter(&zio->io_lock);
 		zio->io_executor = NULL;
 		cv_broadcast(&zio->io_cv);
 		mutex_exit(&zio->io_lock);
 	} else {
+		mutex_exit(&zio->io_lock);
 		zio_destroy(zio);
 	}