block/mirror: change the semantic of 'force' of block-job-cancel

When doing drive mirror to a low speed shared storage, if there was heavy
BLK IO write workload in VM after the 'ready' event, drive mirror block job
can't be canceled immediately, it would keep running until the heavy BLK IO
workload stopped in the VM.

Libvirt depends on the current block-job-cancel semantics, which is that
when used without a flag after the 'ready' event, the command blocks
until data is in sync.  However, these semantics are awkward in other
situations, for example, people may use drive mirror for realtime
backups while still wanting to use block live migration.  Libvirt cannot
start a block live migration while another drive mirror is in progress,
but the user would rather abandon the backup attempt as broken and
proceed with the live migration than be stuck waiting for the current
drive mirror backup to finish.

The drive-mirror command already includes a 'force' flag, which libvirt
does not use, although it documented the flag as only being useful to
quit a job which is paused.  However, since quitting a paused job has
the same effect as abandoning a backup in a non-paused job (namely, the
destination file is not in sync, and the command completes immediately),
we can just improve the documentation to make the force flag obviously
useful.

Cc: Paolo Bonzini <[email protected]>
Cc: Jeff Cody <[email protected]>
Cc: Kevin Wolf <[email protected]>
Cc: Max Reitz <[email protected]>
Cc: Eric Blake <[email protected]>
Cc: John Snow <[email protected]>
Reported-by: Huaitong Han <[email protected]>
Signed-off-by: Huaitong Han <[email protected]>
Signed-off-by: Liang Li <[email protected]>
Signed-off-by: Jeff Cody <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

block/mirror.c

@@ -869,11 +869,8 @@ static void coroutine_fn mirror_run(void *opaque)
 
         ret = 0;
         trace_mirror_before_sleep(s, cnt, s->synced, delay_ns);
-        if (!s->synced) {
-            block_job_sleep_ns(&s->common, delay_ns);
-            if (block_job_is_cancelled(&s->common)) {
-                break;
-            }
+        if (block_job_is_cancelled(&s->common) && s->common.force) {
+            break;
         } else if (!should_complete) {
             delay_ns = (s->in_flight == 0 && cnt == 0 ? SLICE_TIME : 0);
             block_job_sleep_ns(&s->common, delay_ns);
@@ -887,7 +884,8 @@ static void coroutine_fn mirror_run(void *opaque)
          * or it was cancelled prematurely so that we do not guarantee that
          * the target is a copy of the source.
          */
-        assert(ret < 0 || (!s->synced && block_job_is_cancelled(&s->common)));
+        assert(ret < 0 || ((s->common.force || !s->synced) &&
+               block_job_is_cancelled(&s->common)));
         assert(need_drain);
         mirror_wait_for_all_io(s);
     }

blockdev.c

@@ -150,7 +150,7 @@ void blockdev_mark_auto_del(BlockBackend *blk)
         aio_context_acquire(aio_context);
 
         if (bs->job) {
-            block_job_cancel(bs->job);
+            block_job_cancel(bs->job, false);
         }
 
         aio_context_release(aio_context);
@@ -3850,7 +3850,7 @@ void qmp_block_job_cancel(const char *device,
     }
 
     trace_qmp_block_job_cancel(job);
-    block_job_user_cancel(job, errp);
+    block_job_user_cancel(job, force, errp);
 out:
     aio_context_release(aio_context);
 }

blockjob.c

@@ -487,7 +487,7 @@ static int block_job_finalize_single(BlockJob *job)
     return 0;
 }
 
-static void block_job_cancel_async(BlockJob *job)
+static void block_job_cancel_async(BlockJob *job, bool force)
 {
     if (job->iostatus != BLOCK_DEVICE_IO_STATUS_OK) {
         block_job_iostatus_reset(job);
@@ -498,6 +498,8 @@ static void block_job_cancel_async(BlockJob *job)
         job->pause_count--;
     }
     job->cancelled = true;
+    /* To prevent 'force == false' overriding a previous 'force == true' */
+    job->force |= force;
 }
 
 static int block_job_txn_apply(BlockJobTxn *txn, int fn(BlockJob *), bool lock)
@@ -581,7 +583,7 @@ static void block_job_completed_txn_abort(BlockJob *job)
      * on the caller, so leave it. */
     QLIST_FOREACH(other_job, &txn->jobs, txn_list) {
         if (other_job != job) {
-            block_job_cancel_async(other_job);
+            block_job_cancel_async(other_job, false);
         }
     }
     while (!QLIST_EMPTY(&txn->jobs)) {
@@ -747,13 +749,13 @@ void block_job_user_resume(BlockJob *job, Error **errp)
     block_job_resume(job);
 }
 
-void block_job_cancel(BlockJob *job)
+void block_job_cancel(BlockJob *job, bool force)
 {
     if (job->status == BLOCK_JOB_STATUS_CONCLUDED) {
         block_job_do_dismiss(job);
         return;
     }
-    block_job_cancel_async(job);
+    block_job_cancel_async(job, force);
     if (!block_job_started(job)) {
         block_job_completed(job, -ECANCELED);
     } else if (job->deferred_to_main_loop) {
@@ -763,20 +765,20 @@ void block_job_cancel(BlockJob *job)
     }
 }
 
-void block_job_user_cancel(BlockJob *job, Error **errp)
+void block_job_user_cancel(BlockJob *job, bool force, Error **errp)
 {
     if (block_job_apply_verb(job, BLOCK_JOB_VERB_CANCEL, errp)) {
         return;
     }
-    block_job_cancel(job);
+    block_job_cancel(job, force);
 }
 
 /* A wrapper around block_job_cancel() taking an Error ** parameter so it may be
  * used with block_job_finish_sync() without the need for (rather nasty)
  * function pointer casts there. */
 static void block_job_cancel_err(BlockJob *job, Error **errp)
 {
-    block_job_cancel(job);
+    block_job_cancel(job, false);
 }
 
 int block_job_cancel_sync(BlockJob *job)

hmp-commands.hx

@@ -106,7 +106,8 @@ ETEXI
         .args_type  = "force:-f,device:B",
         .params     = "[-f] device",
         .help       = "stop an active background block operation (use -f"
-                      "\n\t\t\t if the operation is currently paused)",
+                      "\n\t\t\t if you want to abort the operation immediately"
+                      "\n\t\t\t instead of keep running until data is in sync)",
         .cmd        = hmp_block_job_cancel,
     },
 

include/block/blockjob.h

@@ -62,6 +62,12 @@ typedef struct BlockJob {
      */
     bool cancelled;
 
+    /**
+     * Set to true if the job should abort immediately without waiting
+     * for data to be in sync.
+     */
+    bool force;
+
     /**
      * Counter for pause request. If non-zero, the block job is either paused,
      * or if busy == true will pause itself as soon as possible.
@@ -230,10 +236,11 @@ void block_job_start(BlockJob *job);
 /**
  * block_job_cancel:
  * @job: The job to be canceled.
+ * @force: Quit a job without waiting for data to be in sync.
  *
  * Asynchronously cancel the specified job.
  */
-void block_job_cancel(BlockJob *job);
+void block_job_cancel(BlockJob *job, bool force);
 
 /**
  * block_job_complete:
@@ -307,11 +314,12 @@ void block_job_user_resume(BlockJob *job, Error **errp);
 /**
  * block_job_user_cancel:
  * @job: The job to be cancelled.
+ * @force: Quit a job without waiting for data to be in sync.
  *
  * Cancels the specified job, but may refuse to do so if the
  * operation isn't currently meaningful.
  */
-void block_job_user_cancel(BlockJob *job, Error **errp);
+void block_job_user_cancel(BlockJob *job, bool force, Error **errp);
 
 /**
  * block_job_cancel_sync:

qapi/block-core.json

@@ -2207,8 +2207,9 @@
 #          the name of the parameter), but since QEMU 2.7 it can have
 #          other values.
 #
-# @force: whether to allow cancellation of a paused job (default
-#         false).  Since 1.3.
+# @force: If true, and the job has already emitted the event BLOCK_JOB_READY,
+#         abandon the job immediately (even if it is paused) instead of waiting
+#         for the destination to complete its final synchronization (since 1.3)
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive

tests/test-blockjob-txn.c

@@ -124,7 +124,7 @@ static void test_single_job(int expected)
     block_job_start(job);
 
     if (expected == -ECANCELED) {
-        block_job_cancel(job);
+        block_job_cancel(job, false);
     }
 
     while (result == -EINPROGRESS) {
@@ -170,10 +170,10 @@ static void test_pair_jobs(int expected1, int expected2)
     block_job_txn_unref(txn);
 
     if (expected1 == -ECANCELED) {
-        block_job_cancel(job1);
+        block_job_cancel(job1, false);
     }
     if (expected2 == -ECANCELED) {
-        block_job_cancel(job2);
+        block_job_cancel(job2, false);
     }
 
     while (result1 == -EINPROGRESS || result2 == -EINPROGRESS) {
@@ -226,7 +226,7 @@ static void test_pair_jobs_fail_cancel_race(void)
     block_job_start(job1);
     block_job_start(job2);
 
-    block_job_cancel(job1);
+    block_job_cancel(job1, false);
 
     /* Now make job2 finish before the main loop kicks jobs.  This simulates
      * the race between a pending kick and another job completing.