[pve-devel] [RFC qemu 2/2] PVE-Backup: Use a transaction to synchronize job states

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

By using a JobTxn, we can sync dirty bitmaps only when *all* jobs were
successful - meaning we don't need to remove them when the backup fails,
since QEMU's BITMAP_SYNC_MODE_ON_SUCCESS will now handle that for us.

To keep the rate-limiting and IO impact from before, we use a sequential
transaction, so drives will still be backed up one after the other.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---
 pve-backup.c | 169 +++++++++++++++------------------------------------
 1 file changed, 50 insertions(+), 119 deletions(-)

diff --git a/pve-backup.c b/pve-backup.c
index 04c21c80aa..ffe3cceef9 100644
--- a/pve-backup.c
+++ b/pve-backup.c
@@ -52,6 +52,7 @@ static struct PVEBackupState {
     VmaWriter *vmaw;
     ProxmoxBackupHandle *pbs;
     GList *di_list;
+    JobTxn *txn;
     QemuMutex backup_mutex;
     CoMutex dump_callback_mutex;
 } backup_state;
@@ -71,32 +72,12 @@ typedef struct PVEBackupDevInfo {
     size_t size;
     uint64_t block_size;
     uint8_t dev_id;
-    bool completed;
     char targetfile[PATH_MAX];
     BdrvDirtyBitmap *bitmap;
     BlockDriverState *target;
+    BlockJob *job;
 } PVEBackupDevInfo;
 
-static void pvebackup_run_next_job(void);
-
-static BlockJob *
-lookup_active_block_job(PVEBackupDevInfo *di)
-{
-    if (!di->completed && di->bs) {
-        for (BlockJob *job = block_job_next(NULL); job; job = block_job_next(job)) {
-            if (job->job.driver->job_type != JOB_TYPE_BACKUP) {
-                continue;
-            }
-
-            BackupBlockJob *bjob = container_of(job, BackupBlockJob, common);
-            if (bjob && bjob->source_bs == di->bs) {
-                return job;
-            }
-        }
-    }
-    return NULL;
-}
-
 static void pvebackup_propagate_error(Error *err)
 {
     qemu_mutex_lock(&backup_state.stat.lock);
@@ -272,18 +253,6 @@ static void coroutine_fn pvebackup_co_cleanup(void *unused)
             if (local_err != NULL) {
                 pvebackup_propagate_error(local_err);
             }
-        } else {
-            // on error or cancel we cannot ensure synchronization of dirty
-            // bitmaps with backup server, so remove all and do full backup next
-            GList *l = backup_state.di_list;
-            while (l) {
-                PVEBackupDevInfo *di = (PVEBackupDevInfo *)l->data;
-                l = g_list_next(l);
-
-                if (di->bitmap) {
-                    bdrv_release_dirty_bitmap(di->bitmap);
-                }
-            }
         }
 
         proxmox_backup_disconnect(backup_state.pbs);
@@ -322,8 +291,6 @@ static void pvebackup_complete_cb(void *opaque, int ret)
 
     qemu_mutex_lock(&backup_state.backup_mutex);
 
-    di->completed = true;
-
     if (ret < 0) {
         Error *local_err = NULL;
         error_setg(&local_err, "job failed with err %d - %s", ret, strerror(-ret));
@@ -336,20 +303,17 @@ static void pvebackup_complete_cb(void *opaque, int ret)
 
     block_on_coroutine_fn(pvebackup_complete_stream, di);
 
-    // remove self from job queue
+    // remove self from job list
     backup_state.di_list = g_list_remove(backup_state.di_list, di);
 
-    if (di->bitmap && ret < 0) {
-        // on error or cancel we cannot ensure synchronization of dirty
-        // bitmaps with backup server, so remove all and do full backup next
-        bdrv_release_dirty_bitmap(di->bitmap);
-    }
-
     g_free(di);
 
-    qemu_mutex_unlock(&backup_state.backup_mutex);
+    /* 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_run_next_job();
+    qemu_mutex_unlock(&backup_state.backup_mutex);
 }
 
 static void pvebackup_cancel(void)
@@ -371,36 +335,28 @@ static void pvebackup_cancel(void)
         proxmox_backup_abort(backup_state.pbs, "backup canceled");
     }
 
+    /* it's enough to cancel one job in the transaction, the rest will follow
+     * automatically */
+    GList *bdi = g_list_first(backup_state.di_list);
+    BlockJob *cancel_job = bdi && bdi->data ?
+        ((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_sync may enter the job, so we need to release the
+     * backup_mutex to avoid deadlock */
     qemu_mutex_unlock(&backup_state.backup_mutex);
 
-    for(;;) {
-
-        BlockJob *next_job = NULL;
-
-        qemu_mutex_lock(&backup_state.backup_mutex);
-
-        GList *l = backup_state.di_list;
-        while (l) {
-            PVEBackupDevInfo *di = (PVEBackupDevInfo *)l->data;
-            l = g_list_next(l);
-
-            BlockJob *job = lookup_active_block_job(di);
-            if (job != NULL) {
-                next_job = job;
-                break;
-            }
-        }
-
-        qemu_mutex_unlock(&backup_state.backup_mutex);
-
-        if (next_job) {
-            AioContext *aio_context = next_job->job.aio_context;
-            aio_context_acquire(aio_context);
-            job_cancel_sync(&next_job->job);
-            aio_context_release(aio_context);
-        } else {
-            break;
-        }
+    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);
     }
 }
 
@@ -459,51 +415,19 @@ static int coroutine_fn pvebackup_co_add_config(
     goto out;
 }
 
-bool job_should_pause(Job *job);
-
-static void pvebackup_run_next_job(void)
-{
-    assert(!qemu_in_coroutine());
-
-    qemu_mutex_lock(&backup_state.backup_mutex);
-
-    GList *l = backup_state.di_list;
-    while (l) {
-        PVEBackupDevInfo *di = (PVEBackupDevInfo *)l->data;
-        l = g_list_next(l);
-
-        BlockJob *job = lookup_active_block_job(di);
-
-        if (job) {
-            qemu_mutex_unlock(&backup_state.backup_mutex);
-
-            AioContext *aio_context = job->job.aio_context;
-            aio_context_acquire(aio_context);
-
-            if (job_should_pause(&job->job)) {
-                bool error_or_canceled = pvebackup_error_or_canceled();
-                if (error_or_canceled) {
-                    job_cancel_sync(&job->job);
-                } else {
-                    job_resume(&job->job);
-                }
-            }
-            aio_context_release(aio_context);
-            return;
-        }
-    }
-
-    block_on_coroutine_fn(pvebackup_co_cleanup, NULL); // no more jobs, run cleanup
-
-    qemu_mutex_unlock(&backup_state.backup_mutex);
-}
-
 static bool create_backup_jobs(void) {
 
     assert(!qemu_in_coroutine());
 
     Error *local_err = NULL;
 
+    /* create job transaction to synchronize bitmap commit and cancel all
+     * jobs in case one errors */
+    if (backup_state.txn) {
+        job_txn_unref(backup_state.txn);
+    }
+    backup_state.txn = job_txn_new_seq();
+
     /* create and start all jobs (paused state) */
     GList *l =  backup_state.di_list;
     while (l) {
@@ -524,7 +448,7 @@ static bool create_backup_jobs(void) {
         BlockJob *job = backup_job_create(
             NULL, di->bs, di->target, backup_state.speed, sync_mode, di->bitmap,
             bitmap_mode, false, NULL, BLOCKDEV_ON_ERROR_REPORT, BLOCKDEV_ON_ERROR_REPORT,
-            JOB_DEFAULT, pvebackup_complete_cb, di, 1, NULL, &local_err);
+            JOB_DEFAULT, pvebackup_complete_cb, di, 0, backup_state.txn, &local_err);
 
         aio_context_release(aio_context);
 
@@ -536,7 +460,8 @@ static bool create_backup_jobs(void) {
             pvebackup_propagate_error(create_job_err);
             break;
         }
-        job_start(&job->job);
+
+        di->job = job;
 
         bdrv_unref(di->target);
         di->target = NULL;
@@ -554,6 +479,10 @@ static bool create_backup_jobs(void) {
                 bdrv_unref(di->target);
                 di->target = NULL;
             }
+
+            if (di->job) {
+                job_unref(&di->job->job);
+            }
         }
     }
 
@@ -944,10 +873,6 @@ err:
         PVEBackupDevInfo *di = (PVEBackupDevInfo *)l->data;
         l = g_list_next(l);
 
-        if (di->bitmap) {
-            bdrv_release_dirty_bitmap(di->bitmap);
-        }
-
         if (di->target) {
             bdrv_unref(di->target);
         }
@@ -1036,9 +961,15 @@ UuidInfo *qmp_backup(
     block_on_coroutine_fn(pvebackup_co_prepare, &task);
 
     if (*errp == NULL) {
-        create_backup_jobs();
+        bool errors = create_backup_jobs();
         qemu_mutex_unlock(&backup_state.backup_mutex);
-        pvebackup_run_next_job();
+
+        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 */
+            job_txn_start_seq(backup_state.txn);
+        }
     } else {
         qemu_mutex_unlock(&backup_state.backup_mutex);
     }
-- 
2.20.1