[PATCH QEMU HCI-8.1 v3 17/28] migration: Fix crash on second migration when cancel early

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

From: Peter Xu <peterx@redhat.com>

Marc-André reported an issue on QEMU crash when retrying a cancelled
migration during early setup phase, see "Link:" for more information, and
also easy way to reproduce.

This patch is a replacement of the prior fix proposed by not only switching
to migration_cleanup(), but also fixing it from CPR side, so that we track
hup_source properly to know if src QEMU is waiting or the HUP signal.

To put it simple: this chunk of special casing in migration_cancel() should
not affect normal migration, but only cpr-transfer migration to cover the
small window when the src QEMU is waiting for a HUP signal on cpr
channel (so that src QEMU can continue the migration on the main channel).

To achieve that, we'll also need to remember to detach the hup_source
whenenver invoked: after that point, we should always be able to cleanup
the migration.

It's not a generic operation to explicitly detach a gsource from its
context while in its dispatch() function.  But it should be safe, because
gsource disptch() will only happen with a boosted refcount for the
dispatcher so that the gsource will not be freed until the callback
completes. It's also safe to return G_SOURCE_REMOVE after the gsource is
detached, as glib will simply ignore the G_SOURCE_REMOVE.

One can refer to latest 2.86.5 glib code in g_main_dispatch() for that:

https://github.com/GNOME/glib/blob/2.86.5/glib/gmain.c#L3592

When at this, add a bunch of assertions to make sure nothing surprises us.

After this patch applied, the 2nd migration will not crash QEMU, instead
it'll be in CANCELLING until the socket connection times out (it will take
~2min on my Fedora default kernel).  During this process no 2nd migration
will be allowed, and after it timed out migration can be restarted.

It's because so far we don't have control over socket_connect_outgoing(),
or anything yet managed by a task executed in qio_task_run_in_thread().
Speeding up the cancellation to be left for future.

I also tested cpr-transfer by only providing cpr channel not the main
channel (with -incoming defer), kickoff migration on source, then cancel it
on source directly without providing the main channel.  It keeps working.

I wanted to add an unit test for that but it'll need to refactor current
cpr-transfer tests first; let's leave it for later.

andrey.drobyshev:
    This commit was cherry-picked from the upstream commit 4f08b870f8f0,
    adapted for the hci-dev base, which predates the upstream qmp_migrate()
    rework: the HUP callback is still qmp_migrate_finish_cb() rather than
    migration_connect_outgoing_cb(), so the early
    cpr_transfer_source_destroy() call lands there, ahead of
    qmp_migrate_finish() and the cpr_state_close() the callback keeps
    doing itself.

Link: https://lore.kernel.org/r/20260417184742.293061-1-marcandre.lureau@redhat.com
Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20260421175820.302795-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/migration/cpr.h  |  1 +
 migration/cpr-transfer.c | 10 ++++++++++
 migration/migration.c    | 32 ++++++++++++++++++++++++--------
 migration/migration.h    |  5 +++++
 4 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/include/migration/cpr.h b/include/migration/cpr.h
index ae441322ff5..4dd397a7c13 100644
--- a/include/migration/cpr.h
+++ b/include/migration/cpr.h
@@ -80,6 +80,7 @@ QEMUFile *cpr_transfer_input(MigrationChannel *channel, Error **errp);
 void cpr_transfer_add_hup_watch(MigrationState *s, QIOChannelFunc func,
                                 void *opaque);
 void cpr_transfer_source_destroy(MigrationState *s);
+bool cpr_transfer_source_active(MigrationState *s);
 
 void cpr_exec_init(void);
 QEMUFile *cpr_exec_output(Error **errp);
diff --git a/migration/cpr-transfer.c b/migration/cpr-transfer.c
index 61d5c9dce2b..9defe7bad78 100644
--- a/migration/cpr-transfer.c
+++ b/migration/cpr-transfer.c
@@ -6,6 +6,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/main-loop.h"
 #include "qapi/clone-visitor.h"
 #include "qapi/error.h"
 #include "qapi/qapi-visit-migration.h"
@@ -79,6 +80,7 @@ QEMUFile *cpr_transfer_input(MigrationChannel *channel, Error **errp)
 void cpr_transfer_add_hup_watch(MigrationState *s, QIOChannelFunc func,
                                 void *opaque)
 {
+    assert(bql_locked());
     s->hup_source = qio_channel_create_watch(cpr_state_ioc(), G_IO_HUP);
     g_source_set_callback(s->hup_source,
                           (GSourceFunc)func,
@@ -89,9 +91,17 @@ void cpr_transfer_add_hup_watch(MigrationState *s, QIOChannelFunc func,
 
 void cpr_transfer_source_destroy(MigrationState *s)
 {
+    assert(bql_locked());
     if (s->hup_source) {
         g_source_destroy(s->hup_source);
         g_source_unref(s->hup_source);
         s->hup_source = NULL;
     }
 }
+
+bool cpr_transfer_source_active(MigrationState *s)
+{
+    /* Whenever the HUP gsource is available, it's active. */
+    assert(bql_locked());
+    return s->hup_source;
+}
diff --git a/migration/migration.c b/migration/migration.c
index c138fa40e10..f48e75e8fad 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1653,14 +1653,19 @@ void migration_cancel(void)
     }
 
     /*
-     * If qmp_migrate_finish has not been called, then there is no path that
-     * will complete the cancellation.  Do it now.
+     * This is cpr-transfer specific processing.
+     *
+     * If this is true, it means cpr-transfer migration is waiting for the
+     * destination to send HUP event on CPR channel to continue the next
+     * phase.  If so, do the cleanup proactively to avoid get stuck in
+     * CANCELLING state.
      */
-    if (setup && !s->to_dst_file) {
-        migrate_set_state(&s->state, MIGRATION_STATUS_CANCELLING,
-                          MIGRATION_STATUS_CANCELLED);
-        cpr_state_close();
-        cpr_transfer_source_destroy(s);
+    if (cpr_transfer_source_active(s)) {
+        assert(migrate_mode() == MIG_MODE_CPR_TRANSFER);
+        assert(setup && !s->to_dst_file);
+        migration_cleanup(s);
+        /* Now all things should have been released */
+        assert(!cpr_transfer_source_active(s));
     }
 }
 
@@ -2255,11 +2260,22 @@ static gboolean qmp_migrate_finish_cb(QIOChannel *channel,
                                       void *opaque)
 {
     MigrationAddress *addr = opaque;
+    MigrationState *s = migrate_get_current();
+
+    /*
+     * Detach and release the GSource right after use.  We rely on this to
+     * detect this small cpr-transfer window of "waiting for HUP event".
+     */
+    cpr_transfer_source_destroy(s);
 
     qmp_migrate_finish(addr, NULL);
 
     cpr_state_close();
-    cpr_transfer_source_destroy(migrate_get_current());
+
+    /*
+     * This is redundant as we do cpr_transfer_source_destroy() at the
+     * entry, but it's benign; glib will just skip the detach.
+     */
     return G_SOURCE_REMOVE;
 }
 
diff --git a/migration/migration.h b/migration/migration.h
index a7ea43e925d..ee15ca88461 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -509,6 +509,11 @@ struct MigrationState {
     /* Is this a rdma migration */
     bool rdma_migration;
 
+    /*
+     * When set, it means cpr-transfer is waiting for the HUP signal from
+     * destination to continue the 2nd step of migration via the main
+     * channel.
+     */
     GSource *hup_source;
 };
 

-- 
2.47.1