[pve-devel] [PATCH qemu 4/5] PVE-Backup: Use more coroutines and don't block on finishing

[Stefan Reiter <s.reiter@proxmox.com>]

proxmox_backup_co_finish is already async, but previously we would wait
for the coroutine using block_on_coroutine_fn(). Avoid this by
scheduling pvebackup_co_complete_stream (and thus pvebackup_co_cleanup)
as a real coroutine when calling from pvebackup_complete_cb. This is ok,
since complete_stream uses the backup_mutex internally to synchronize,
and other streams can happily continue writing in the meantime anyway.

To accomodate, backup_mutex is converted to a CoMutex. This means
converting every user to a coroutine. This is not just useful here, but
will come in handy once this series[0] is merged, and QMP calls can be
yield-able coroutines too. Then we can also finally get rid of
block_on_coroutine_fn.

Cases of aio_context_acquire/release from within what is now a coroutine
are changed to aio_co_reschedule_self, which works since a running
coroutine always holds the aio lock for the context it is running in.

job_cancel_sync is changed to regular job_cancel, since job_cancel_sync
uses AIO_WAIT_WHILE internally, which is forbidden from Coroutines.

create_backup_jobs cannot be run from a coroutine, so it is run
directly. This does however mean that it runs unlocked, as it cannot
acquire a CoMutex (see it's comment for rational on why that's ok).

To communicate the finishing state, a new property is introduced to
query-backup: 'finishing'. A new state is explicitly not used, since
that would break compatibility with older qemu-server versions.

[0] https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg03515.html

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---
 proxmox-backup-client.h |   1 +
 pve-backup.c            | 124 ++++++++++++++++++++++------------------
 qapi/block-core.json    |   5 +-
 3 files changed, 74 insertions(+), 56 deletions(-)

diff --git a/proxmox-backup-client.h b/proxmox-backup-client.h
index 20fd6b1719..a4781c5851 100644
--- a/proxmox-backup-client.h
+++ b/proxmox-backup-client.h
@@ -5,6 +5,7 @@
 #include "qemu/coroutine.h"
 #include "proxmox-backup-qemu.h"
 
+// FIXME: Remove once coroutines are supported for QMP
 void block_on_coroutine_fn(CoroutineEntry *entry, void *entry_arg);
 
 int coroutine_fn
diff --git a/pve-backup.c b/pve-backup.c
index 9562e9c98d..53cf23ed5a 100644
--- a/pve-backup.c
+++ b/pve-backup.c
@@ -33,7 +33,9 @@ const char *PBS_BITMAP_NAME = "pbs-incremental-dirty-bitmap";
 
 static struct PVEBackupState {
     struct {
-        // Everithing accessed from qmp_backup_query command is protected using lock
+        // Everything accessed from qmp_backup_query command is protected using
+        // this lock. Do NOT hold this lock for long times, as it is sometimes
+        // acquired from coroutines, and thus any wait time may block the guest.
         QemuMutex lock;
         Error *error;
         time_t start_time;
@@ -47,20 +49,21 @@ static struct PVEBackupState {
         size_t reused;
         size_t zero_bytes;
         GList *bitmap_list;
+        bool finishing;
     } stat;
     int64_t speed;
     VmaWriter *vmaw;
     ProxmoxBackupHandle *pbs;
     GList *di_list;
     JobTxn *txn;
-    QemuMutex backup_mutex;
+    CoMutex backup_mutex;
     CoMutex dump_callback_mutex;
 } backup_state;
 
 static void pvebackup_init(void)
 {
     qemu_mutex_init(&backup_state.stat.lock);
-    qemu_mutex_init(&backup_state.backup_mutex);
+    qemu_co_mutex_init(&backup_state.backup_mutex);
     qemu_co_mutex_init(&backup_state.dump_callback_mutex);
 }
 
@@ -72,6 +75,7 @@ typedef struct PVEBackupDevInfo {
     size_t size;
     uint64_t block_size;
     uint8_t dev_id;
+    int completed_ret; // INT_MAX if not completed
     char targetfile[PATH_MAX];
     BdrvDirtyBitmap *bitmap;
     BlockDriverState *target;
@@ -227,12 +231,12 @@ pvebackup_co_dump_vma_cb(
 }
 
 // assumes the caller holds backup_mutex
-static void coroutine_fn pvebackup_co_cleanup(void *unused)
+static void coroutine_fn pvebackup_co_cleanup(void)
 {
     assert(qemu_in_coroutine());
 
     qemu_mutex_lock(&backup_state.stat.lock);
-    backup_state.stat.end_time = time(NULL);
+    backup_state.stat.finishing = true;
     qemu_mutex_unlock(&backup_state.stat.lock);
 
     if (backup_state.vmaw) {
@@ -261,12 +265,29 @@ static void coroutine_fn pvebackup_co_cleanup(void *unused)
 
     g_list_free(backup_state.di_list);
     backup_state.di_list = NULL;
+
+    qemu_mutex_lock(&backup_state.stat.lock);
+    backup_state.stat.end_time = time(NULL);
+    backup_state.stat.finishing = false;
+    qemu_mutex_unlock(&backup_state.stat.lock);
 }
 
-// assumes the caller holds backup_mutex
-static void coroutine_fn pvebackup_complete_stream(void *opaque)
+static void coroutine_fn pvebackup_co_complete_stream(void *opaque)
 {
     PVEBackupDevInfo *di = opaque;
+    int ret = di->completed_ret;
+
+    qemu_co_mutex_lock(&backup_state.backup_mutex);
+
+    if (ret < 0) {
+        Error *local_err = NULL;
+        error_setg(&local_err, "job failed with err %d - %s", ret, strerror(-ret));
+        pvebackup_propagate_error(local_err);
+    }
+
+    di->bs = NULL;
+
+    assert(di->target == NULL);
 
     bool error_or_canceled = pvebackup_error_or_canceled();
 
@@ -281,27 +302,6 @@ static void coroutine_fn pvebackup_complete_stream(void *opaque)
             pvebackup_propagate_error(local_err);
         }
     }
-}
-
-static void pvebackup_complete_cb(void *opaque, int ret)
-{
-    assert(!qemu_in_coroutine());
-
-    PVEBackupDevInfo *di = opaque;
-
-    qemu_mutex_lock(&backup_state.backup_mutex);
-
-    if (ret < 0) {
-        Error *local_err = NULL;
-        error_setg(&local_err, "job failed with err %d - %s", ret, strerror(-ret));
-        pvebackup_propagate_error(local_err);
-    }
-
-    di->bs = NULL;
-
-    assert(di->target == NULL);
-
-    block_on_coroutine_fn(pvebackup_complete_stream, di);
 
     // remove self from job list
     backup_state.di_list = g_list_remove(backup_state.di_list, di);
@@ -310,21 +310,36 @@ static void pvebackup_complete_cb(void *opaque, int ret)
 
     /* call cleanup if we're the last job */
     if (!g_list_first(backup_state.di_list)) {
-        block_on_coroutine_fn(pvebackup_co_cleanup, NULL);
+        pvebackup_co_cleanup();
     }
 
-    qemu_mutex_unlock(&backup_state.backup_mutex);
+    qemu_co_mutex_unlock(&backup_state.backup_mutex);
 }
 
-static void pvebackup_cancel(void)
+static void pvebackup_complete_cb(void *opaque, int ret)
 {
     assert(!qemu_in_coroutine());
 
+    PVEBackupDevInfo *di = opaque;
+    di->completed_ret = ret;
+
+    /*
+     * Schedule stream cleanup in async coroutine. close_image and finish might
+     * take a while, so we can't block on them here.
+     * Note: di is a pointer to an entry in the global backup_state struct, so
+     * it stays valid.
+     */
+    Coroutine *co = qemu_coroutine_create(pvebackup_co_complete_stream, di);
+    aio_co_schedule(qemu_get_aio_context(), co);
+}
+
+static void coroutine_fn pvebackup_co_cancel(void *opaque)
+{
     Error *cancel_err = NULL;
     error_setg(&cancel_err, "backup canceled");
     pvebackup_propagate_error(cancel_err);
 
-    qemu_mutex_lock(&backup_state.backup_mutex);
+    qemu_co_mutex_lock(&backup_state.backup_mutex);
 
     if (backup_state.vmaw) {
         /* make sure vma writer does not block anymore */
@@ -342,27 +357,16 @@ static void pvebackup_cancel(void)
         ((PVEBackupDevInfo *)bdi->data)->job :
         NULL;
 
-    /* ref the job before releasing the mutex, just to be safe */
     if (cancel_job) {
-        job_ref(&cancel_job->job);
+        job_cancel(&cancel_job->job, false);
     }
 
-    /* job_cancel_sync may enter the job, so we need to release the
-     * backup_mutex to avoid deadlock */
-    qemu_mutex_unlock(&backup_state.backup_mutex);
-
-    if (cancel_job) {
-        AioContext *aio_context = cancel_job->job.aio_context;
-        aio_context_acquire(aio_context);
-        job_cancel_sync(&cancel_job->job);
-        job_unref(&cancel_job->job);
-        aio_context_release(aio_context);
-    }
+    qemu_co_mutex_unlock(&backup_state.backup_mutex);
 }
 
 void qmp_backup_cancel(Error **errp)
 {
-    pvebackup_cancel();
+    block_on_coroutine_fn(pvebackup_co_cancel, NULL);
 }
 
 // assumes the caller holds backup_mutex
@@ -415,6 +419,14 @@ static int coroutine_fn pvebackup_co_add_config(
     goto out;
 }
 
+/*
+ * backup_job_create can *not* be run from a coroutine (and requires an
+ * acquired AioContext), so this can't either.
+ * This does imply that this function cannot run with backup_mutex acquired.
+ * That is ok because it is only ever called between setting up the backup_state
+ * struct and starting the jobs, and from within a QMP call. This means that no
+ * other QMP call can interrupt, and no background job is running yet.
+ */
 static bool create_backup_jobs(void) {
 
     assert(!qemu_in_coroutine());
@@ -523,11 +535,12 @@ typedef struct QmpBackupTask {
     UuidInfo *result;
 } QmpBackupTask;
 
-// assumes the caller holds backup_mutex
 static void coroutine_fn pvebackup_co_prepare(void *opaque)
 {
     assert(qemu_in_coroutine());
 
+    qemu_co_mutex_lock(&backup_state.backup_mutex);
+
     QmpBackupTask *task = opaque;
 
     task->result = NULL; // just to be sure
@@ -616,6 +629,8 @@ static void coroutine_fn pvebackup_co_prepare(void *opaque)
         }
         di->size = size;
         total += size;
+
+        di->completed_ret = INT_MAX;
     }
 
     uuid_generate(uuid);
@@ -847,6 +862,7 @@ static void coroutine_fn pvebackup_co_prepare(void *opaque)
     backup_state.stat.dirty = total - backup_state.stat.reused;
     backup_state.stat.transferred = 0;
     backup_state.stat.zero_bytes = 0;
+    backup_state.stat.finishing = false;
 
     qemu_mutex_unlock(&backup_state.stat.lock);
 
@@ -861,6 +877,8 @@ static void coroutine_fn pvebackup_co_prepare(void *opaque)
     uuid_info->UUID = uuid_str;
 
     task->result = uuid_info;
+
+    qemu_co_mutex_unlock(&backup_state.backup_mutex);
     return;
 
 err_mutex:
@@ -903,6 +921,8 @@ err:
     }
 
     task->result = NULL;
+
+    qemu_co_mutex_unlock(&backup_state.backup_mutex);
     return;
 }
 
@@ -956,22 +976,15 @@ UuidInfo *qmp_backup(
         .errp = errp,
     };
 
-    qemu_mutex_lock(&backup_state.backup_mutex);
-
     block_on_coroutine_fn(pvebackup_co_prepare, &task);
 
     if (*errp == NULL) {
         bool errors = create_backup_jobs();
-        qemu_mutex_unlock(&backup_state.backup_mutex);
 
         if (!errors) {
-            /* start the first job in the transaction
-             * note: this might directly enter the job, so we need to do this
-             * after unlocking the backup_mutex */
+            // start the first job in the transaction
             job_txn_start_seq(backup_state.txn);
         }
-    } else {
-        qemu_mutex_unlock(&backup_state.backup_mutex);
     }
 
     return task.result;
@@ -1025,6 +1038,7 @@ BackupStatus *qmp_query_backup(Error **errp)
     info->transferred = backup_state.stat.transferred;
     info->has_reused = true;
     info->reused = backup_state.stat.reused;
+    info->finishing = backup_state.stat.finishing;
 
     qemu_mutex_unlock(&backup_state.stat.lock);
 
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 5fc42e87f3..b31ad8d989 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -784,12 +784,15 @@
 #
 # @uuid: uuid for this backup job
 #
+# @finishing: if status='active' and finishing=true, then the backup process is
+#             waiting for the target to finish.
+#
 ##
 { 'struct': 'BackupStatus',
   'data': {'*status': 'str', '*errmsg': 'str', '*total': 'int', '*dirty': 'int',
            '*transferred': 'int', '*zero-bytes': 'int', '*reused': 'int',
            '*start-time': 'int', '*end-time': 'int',
-           '*backup-file': 'str', '*uuid': 'str' } }
+           '*backup-file': 'str', '*uuid': 'str', 'finishing': 'bool' } }
 
 ##
 # @BackupFormat:
-- 
2.20.1