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vmm: prevent serial manager TCP shutdown deadlock #172

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Merged
amphi merged 1 commit into from
Jun 18, 2026
Merged

vmm: prevent serial manager TCP shutdown deadlock #172

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16 changes: 16 additions & 0 deletions vmm/src/serial_manager.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -176,6 +176,8 @@ pub struct SerialManager {
handle: Option<thread::JoinHandle<()>>,
pty_write_out: Option<Arc<AtomicBool>>,
socket_path: Option<PathBuf>,
/// Tracks the active TCP client so `Drop` can shut it down before joining.
active_tcp_stream: Arc<Mutex<Option<TcpStream>>>,

@phip1611 phip1611 Jun 18, 2026 •
edited
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I think this should be Option<Arc<Mutex<TcpStream>>>. I'm unsure if you need the Arc<Mutex> at all since you seem to have &mut self available on every call site. Would Option<TcpStream> also work?

@amphi amphi Jun 18, 2026

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The variable is shared between the main VMM thread and serial-manager thread, so I think we need at least the Arc, and I'd like to synchronize the access to it, thus the Mutex

@phip1611 phip1611 Jun 18, 2026 •
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and I'd like to synchronize the access to it, thus the Mutex

Never use a Mutex because you'd like to synchronize access - the compiler either forces you or doesn't complain which means there are no unsynchronized accesses :)

But I assume you wanted to say that Rust complains without Arc<Mutex<>>, hence the Mutex is necessary?

@amphi amphi Jun 18, 2026

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I just tried to remove the Mutex<> part, but yes the compiler complains without it.

}

impl SerialManager {
Expand Down Expand Up @@ -278,6 +280,7 @@ impl SerialManager {
handle: None,
pty_write_out,
socket_path,
active_tcp_stream: Arc::new(Mutex::new(None)),
}))
}

Expand Down Expand Up @@ -320,6 +323,9 @@ impl SerialManager {
let pty_write_out = self.pty_write_out.clone();
let mut reader: Option<UnixStream> = None;
let mut reader_tcp: Option<TcpStream> = None;
// Keep `active_tcp_stream` and `reader_tcp` in sync: the former is the
// shutdown handle, the latter is the worker's read-side owner.
let active_tcp_stream = self.active_tcp_stream.clone();

// In case of PTY, we want to be able to detect a connection on the
// other end of the PTY. This is done by detecting there's no event
Expand Down Expand Up @@ -428,6 +434,7 @@ impl SerialManager {
previous_reader
.shutdown(Shutdown::Both)
.map_err(Error::AcceptConnection)?;
active_tcp_stream.lock().unwrap().take();
if let Some(distributor) = &write_distributor {
distributor.remove_writer(TypeId::of::<TcpStream>());
}
Expand All @@ -441,6 +448,8 @@ impl SerialManager {
// Accept them, create a reader and a writer.
let (tcp_stream, _) =
listener.accept().map_err(Error::AcceptConnection)?;
let active_tcp_stream_handle =
tcp_stream.try_clone().map_err(Error::CloneStream)?;
let writer =
tcp_stream.try_clone().map_err(Error::CloneStream)?;

Expand All @@ -454,6 +463,8 @@ impl SerialManager {
),
)
.map_err(Error::Epoll)?;
*active_tcp_stream.lock().unwrap() =
Some(active_tcp_stream_handle);
reader_tcp = Some(tcp_stream);
if let Some(distributor) = &write_distributor {
distributor.add_writer(writer);
Expand Down Expand Up @@ -497,6 +508,7 @@ impl SerialManager {
.shutdown(Shutdown::Both)
.map_err(Error::ShutdownConnection)?;
reader_tcp = None;
active_tcp_stream.lock().unwrap().take();
if let Some(distributor) =
&write_distributor
{
Expand Down Expand Up @@ -566,6 +578,10 @@ impl SerialManager {

impl Drop for SerialManager {
fn drop(&mut self) {
if let Some(tcp_stream) = self.active_tcp_stream.lock().unwrap().take() {
tcp_stream.shutdown(Shutdown::Both).ok();
}

self.kill_evt.write(1).ok();
if let Some(handle) = self.handle.take() {
handle.join().ok();
Expand Down
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