[Qemu-devel] [PATCH v10 15/24] migration: Synchronize send threads

[Juan Quintela <quintela@redhat.com>]

We synchronize all threads each RAM_SAVE_FLAG_EOS.  Bitmap
synchronizations don't happen inside a  ram section, so we are safe
about two channels trying to overwrite the same memory.

Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 migration/ram.c        | 38 +++++++++++++++++++++++++++++++++++++-
 migration/trace-events |  1 +
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/migration/ram.c b/migration/ram.c
index e502be5dda..153c7560cb 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -403,6 +403,7 @@ struct MultiFDSendParams {
     QemuMutex mutex;
     bool running;
     bool quit;
+    bool sync;
 };
 typedef struct MultiFDSendParams MultiFDSendParams;
 
@@ -410,6 +411,8 @@ struct {
     MultiFDSendParams *params;
     /* number of created threads */
     int count;
+    /* syncs main thread and channels */
+    QemuSemaphore sem_main;
 } *multifd_send_state;
 
 static void terminate_multifd_send_threads(Error *errp)
@@ -457,6 +460,7 @@ int multifd_save_cleanup(Error **errp)
         g_free(p->name);
         p->name = NULL;
     }
+    qemu_sem_destroy(&multifd_send_state->sem_main);
     g_free(multifd_send_state->params);
     multifd_send_state->params = NULL;
     g_free(multifd_send_state);
@@ -464,18 +468,44 @@ int multifd_save_cleanup(Error **errp)
     return ret;
 }
 
+static void multifd_send_sync_main(void)
+{
+    int i;
+
+    if (!migrate_use_multifd()) {
+        return;
+    }
+    for (i = 0; i < migrate_multifd_channels(); i++) {
+        MultiFDSendParams *p = &multifd_send_state->params[i];
+        qemu_mutex_lock(&p->mutex);
+        p->sync = true;
+        qemu_mutex_unlock(&p->mutex);
+        qemu_sem_post(&p->sem);
+    }
+    for (i = 0; i < migrate_multifd_channels(); i++) {
+        qemu_sem_wait(&multifd_send_state->sem_main);
+    }
+    trace_multifd_send_sync_main();
+}
+
 static void *multifd_send_thread(void *opaque)
 {
     MultiFDSendParams *p = opaque;
 
     while (true) {
+        qemu_sem_wait(&p->sem);
         qemu_mutex_lock(&p->mutex);
+        if (p->sync) {
+            p->sync = false;
+            qemu_mutex_unlock(&p->mutex);
+            qemu_sem_post(&multifd_send_state->sem_main);
+            continue;
+        }
         if (p->quit) {
             qemu_mutex_unlock(&p->mutex);
             break;
         }
         qemu_mutex_unlock(&p->mutex);
-        qemu_sem_wait(&p->sem);
     }
 
     return NULL;
@@ -493,6 +523,8 @@ int multifd_save_setup(void)
     multifd_send_state = g_malloc0(sizeof(*multifd_send_state));
     multifd_send_state->params = g_new0(MultiFDSendParams, thread_count);
     atomic_set(&multifd_send_state->count, 0);
+    qemu_sem_init(&multifd_send_state->sem_main, 0);
+
     for (i = 0; i < thread_count; i++) {
         MultiFDSendParams *p = &multifd_send_state->params[i];
 
@@ -507,6 +539,7 @@ int multifd_save_setup(void)
 
         atomic_inc(&multifd_send_state->count);
     }
+
     return 0;
 }
 
@@ -2283,6 +2316,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     ram_control_before_iterate(f, RAM_CONTROL_SETUP);
     ram_control_after_iterate(f, RAM_CONTROL_SETUP);
 
+    multifd_send_sync_main();
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
 
     return 0;
@@ -2351,6 +2385,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
      */
     ram_control_after_iterate(f, RAM_CONTROL_ROUND);
 
+    multifd_send_sync_main();
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
     ram_counters.transferred += 8;
 
@@ -2403,6 +2438,7 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
 
     rcu_read_unlock();
 
+    multifd_send_sync_main();
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
 
     return 0;
diff --git a/migration/trace-events b/migration/trace-events
index 93961dea16..97b5ac564f 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -77,6 +77,7 @@ ram_load_postcopy_loop(uint64_t addr, int flags) "@%" PRIx64 " %x"
 ram_postcopy_send_discard_bitmap(void) ""
 ram_save_page(const char *rbname, uint64_t offset, void *host) "%s: offset: 0x%" PRIx64 " host: %p"
 ram_save_queue_pages(const char *rbname, size_t start, size_t len) "%s: start: 0x%zx len: 0x%zx"
+multifd_send_sync_main(void) ""
 
 # migration/migration.c
 await_return_path_close_on_source_close(void) ""
-- 
2.14.3