[PATCH 1/1] migration: Terminate multifd threads on yank

[Leonardo Bras]

>From source host viewpoint, losing a connection during migration will
cause the sockets to get stuck in sendmsg() syscall, waiting for
the receiving side to reply.

In migration, yank works by shutting-down the migration QIOChannel fd.
This causes a failure in the next sendmsg() for that fd, and the whole
migration gets cancelled.

In multifd, due to having multiple sockets in multiple threads,
on a connection loss there will be extra sockets stuck in sendmsg(),
and because they will be holding their own mutex, there is good chance
the main migration thread can get stuck in multifd_send_pages()
waiting for one of those mutexes.

While it's waiting, the main migration thread can't run sendmsg() on
it's fd, and therefore can't cause the migration to be cancelled, thus
causing yank not to work.

Fixes this by shutting down all migration fds (including multifd ones),
so no thread get's stuck in sendmsg() while holding a lock, and thus
allowing the main migration thread to properly cancel migration when
yank is used.

There is no need to do the same procedure to yank to work in the
receiving host since ops->recv_pages() is kept outside the mutex protected
code in multifd_recv_thread().

Buglink:https://bugzilla.redhat.com/show_bug.cgi?id=1970337
Reported-by: Li Xiaohui <xiaohli@redhat.com>
Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
 migration/multifd.c        | 11 +++++++++++
 migration/multifd.h        |  1 +
 migration/yank_functions.c |  2 ++
 3 files changed, 14 insertions(+)

diff --git a/migration/multifd.c b/migration/multifd.c
index 377da78f5b..744a180dfe 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -1040,6 +1040,17 @@ void multifd_recv_sync_main(void)
     trace_multifd_recv_sync_main(multifd_recv_state->packet_num);
 }
 
+void multifd_shutdown(void)
+{
+    if (!migrate_use_multifd()) {
+        return;
+    }
+
+    if (multifd_send_state) {
+        multifd_send_terminate_threads(NULL);
+    }
+}
+
 static void *multifd_recv_thread(void *opaque)
 {
     MultiFDRecvParams *p = opaque;
diff --git a/migration/multifd.h b/migration/multifd.h
index 8d6751f5ed..0517213bdf 100644
--- a/migration/multifd.h
+++ b/migration/multifd.h
@@ -22,6 +22,7 @@ bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp);
 void multifd_recv_sync_main(void);
 void multifd_send_sync_main(QEMUFile *f);
 int multifd_queue_page(QEMUFile *f, RAMBlock *block, ram_addr_t offset);
+void multifd_shutdown(void);
 
 /* Multifd Compression flags */
 #define MULTIFD_FLAG_SYNC (1 << 0)
diff --git a/migration/yank_functions.c b/migration/yank_functions.c
index 8c08aef14a..9335a64f00 100644
--- a/migration/yank_functions.c
+++ b/migration/yank_functions.c
@@ -15,12 +15,14 @@
 #include "io/channel-socket.h"
 #include "io/channel-tls.h"
 #include "qemu-file.h"
+#include "multifd.h"
 
 void migration_yank_iochannel(void *opaque)
 {
     QIOChannel *ioc = QIO_CHANNEL(opaque);
 
     qio_channel_shutdown(ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+    multifd_shutdown();
 }
 
 /* Return whether yank is supported on this ioc */
-- 
2.32.0