On Wed, Jan 08, 2025 at 19:42:56 +0000, Daniel P. Berrangé wrote:
> The preserving of state (ie running VMs) requires a fully functional
> daemon and hypervisor driver. If any part has started shutting down
> then saving state may fail, or worse, hang.
>
> The current shutdown sequence does not guarantee safe ordering, as
> we synchronize with the state saving thread only after the hypervisor
> driver has had its 'shutdownPrepare' callback invoked. In the case of
> QEMU this means that worker threads processing monitor events may well
> have been stopped.
>
> This implements a full state machine that has a well defined ordering
> that an earlier commit documented as the desired semantics.
>
> With this change, nothing will start shutting down if the state saving
> thread is still running.
>
> Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
> ---
> src/remote/remote_daemon.c | 4 +-
> src/rpc/virnetdaemon.c | 103 ++++++++++++++++++++++++++-----------
> 2 files changed, 76 insertions(+), 31 deletions(-)
>
> diff --git a/src/remote/remote_daemon.c b/src/remote/remote_daemon.c
> index e03ee1de5a..f64ecad7f5 100644
> --- a/src/remote/remote_daemon.c
> +++ b/src/remote/remote_daemon.c
> @@ -617,8 +617,8 @@ static void daemonRunStateInit(void *opaque)
> NULL,
> G_DBUS_SIGNAL_FLAGS_NONE,
> handleSystemMessageFunc,
> - dmn,
> - NULL);
> + dmn,
> + NULL);
>
> /* Only now accept clients from network */
> virNetDaemonUpdateServices(dmn, true);
This belongs to a different commit.
> diff --git a/src/rpc/virnetdaemon.c b/src/rpc/virnetdaemon.c
> index 7f14c5420a..d1d7bae569 100644
> --- a/src/rpc/virnetdaemon.c
> +++ b/src/rpc/virnetdaemon.c
> @@ -39,6 +39,15 @@
>
> VIR_LOG_INIT("rpc.netdaemon");
>
> +typedef enum {
> + VIR_NET_DAEMON_QUIT_NONE,
> + VIR_NET_DAEMON_QUIT_REQUESTED,
> + VIR_NET_DAEMON_QUIT_PRESERVING,
> + VIR_NET_DAEMON_QUIT_READY,
> + VIR_NET_DAEMON_QUIT_WAITING,
> + VIR_NET_DAEMON_QUIT_COMPLETED,
> +} virNetDaemonQuitPhase;
[...]
> @@ -753,7 +762,7 @@ virNetDaemonRun(virNetDaemon *dmn)
> virSystemdNotifyReady();
>
> VIR_DEBUG("dmn=%p quit=%d", dmn, dmn->quit);
> - while (!dmn->finished) {
> + while (dmn->quit != VIR_NET_DAEMON_QUIT_COMPLETED) {
> virNetDaemonShutdownTimerUpdate(dmn);
>
> virObjectUnlock(dmn);
> @@ -767,17 +776,30 @@ virNetDaemonRun(virNetDaemon *dmn)
> virHashForEach(dmn->servers, daemonServerProcessClients, NULL);
>
> /* don't shutdown services when performing an exec-restart */
> - if (dmn->quit && dmn->execRestart)
> + if (dmn->quit == VIR_NET_DAEMON_QUIT_REQUESTED && dmn->execRestart)
> goto cleanup;
>
> - if (dmn->quit && dmn->finishTimer == -1) {
> + if (dmn->quit == VIR_NET_DAEMON_QUIT_REQUESTED) {
> + VIR_DEBUG("Process quit request");
> virHashForEach(dmn->servers, daemonServerClose, NULL);
> +
> + if (dmn->shutdownPreserveThread) {
> + VIR_DEBUG("Shutdown preserve thread running");
> + dmn->quit = VIR_NET_DAEMON_QUIT_PRESERVING;
> + } else {
> + VIR_DEBUG("Ready to shutdown");
> + dmn->quit = VIR_NET_DAEMON_QUIT_READY;
> + }
> + }
> +
> + if (dmn->quit == VIR_NET_DAEMON_QUIT_READY) {
> + VIR_DEBUG("Starting shutdown, running prepare");
> if (dmn->shutdownPrepareCb && dmn->shutdownPrepareCb() < 0)
> break;
>
> - if ((dmn->finishTimer = virEventAddTimeout(30 * 1000,
> - virNetDaemonFinishTimer,
> - dmn, NULL)) < 0) {
> + if ((dmn->quitTimer = virEventAddTimeout(30 * 1000,
> + virNetDaemonQuitTimer,
> + dmn, NULL)) < 0) {
> VIR_WARN("Failed to register finish timer.");
> break;
> }
> @@ -787,6 +809,9 @@ virNetDaemonRun(virNetDaemon *dmn)
> VIR_WARN("Failed to register join thread.");
> break;
> }
> +
> + VIR_DEBUG("Waiting for shutdown completion");
> + dmn->quit = VIR_NET_DAEMON_QUIT_WAITING;
[1]
> @@ -808,7 +833,7 @@ virNetDaemonQuit(virNetDaemon *dmn)
> VIR_LOCK_GUARD lock = virObjectLockGuard(dmn);
>
> VIR_DEBUG("Quit requested %p", dmn);
> - dmn->quit = true;
> + dmn->quit = VIR_NET_DAEMON_QUIT_REQUESTED;
> }
>
>
> @@ -818,7 +843,7 @@ virNetDaemonQuitExecRestart(virNetDaemon *dmn)
> VIR_LOCK_GUARD lock = virObjectLockGuard(dmn);
>
> VIR_DEBUG("Exec-restart requested %p", dmn);
> - dmn->quit = true;
> + dmn->quit = VIR_NET_DAEMON_QUIT_REQUESTED;
> dmn->execRestart = true;
> }
These two seem to be apart from other cases also be registered in
various signal (SIGTERM/SIGINT/etc) handlers. As the signal handler code
we use doesn't seem to interlock multiple deliveries of said signals it
seems that these will be able to mess up the state of the state machine
if a 'well-timed' signal hits. Specifically they could clear
VIR_NET_DAEMON_QUIT_PRESERVING or VIR_NET_DAEMON_QUIT_WAITING.
When VIR_NET_DAEMON_QUIT_PRESERVING will be overwritten it will get
fixed in the next iteration of the worker
loop in virNetDaemonRun(), but will re-notify systemd about the shutdown
and re-stop the 'server's.
What's most likely worse is that [1] VIR_NET_DAEMON_QUIT_WAITING can
be lost as well, which would re-register the timer and start another
shutdown thread.
>
> @@ -827,12 +852,21 @@ static void virNetDaemonPreserveWorker(void *opaque)
> {
> virNetDaemon *dmn = opaque;
>
> - VIR_DEBUG("Begin stop dmn=%p", dmn);
> + VIR_DEBUG("Begin preserve dmn=%p", dmn);
>
> dmn->shutdownPreserveCb();
>
> - VIR_DEBUG("Completed stop dmn=%p", dmn);
> + VIR_DEBUG("Completed preserve dmn=%p", dmn);
>
> + VIR_WITH_OBJECT_LOCK_GUARD(dmn) {
> + if (dmn->quit == VIR_NET_DAEMON_QUIT_PRESERVING) {
> + VIR_DEBUG("Marking shutdown as ready");
> + dmn->quit = VIR_NET_DAEMON_QUIT_READY;
> + }
> + g_clear_pointer(&dmn->shutdownPreserveThread, g_free);
> + }
> +
> + VIR_DEBUG("End preserve dmn=%p", dmn);
> virObjectUnref(dmn);
> }
>
> @@ -840,15 +874,26 @@ static void virNetDaemonPreserveWorker(void *opaque)
> void virNetDaemonPreserve(virNetDaemon *dmn)
> {
> VIR_LOCK_GUARD lock = virObjectLockGuard(dmn);
> + VIR_DEBUG("Preserve state request");
>
> - if (!dmn->shutdownPreserveCb ||
> - dmn->shutdownPreserveThread)
> + if (!dmn->shutdownPreserveCb) {
> + VIR_DEBUG("No preserve callback registered");
> return;
> + }
> + if (dmn->shutdownPreserveThread) {
> + VIR_DEBUG("Preserve state thread already running");
> + return;
> + }
> +
> + if (dmn->quit != VIR_NET_DAEMON_QUIT_NONE) {
> + VIR_WARN("Already initiated shutdown sequence, unable to preserve state");
> + return;
> + }
>
> virObjectRef(dmn);
> dmn->shutdownPreserveThread = g_new0(virThread, 1);
>
> - if (virThreadCreateFull(dmn->shutdownPreserveThread, true,
> + if (virThreadCreateFull(dmn->shutdownPreserveThread, false,
This looks like a bugfix to a previous commit as the thread was never
actually joined.
> virNetDaemonPreserveWorker,
> "daemon-stop", false, dmn) < 0) {
> virObjectUnref(dmn);
