[PATCH v1] migration: refactor migration_completion

[PATCH v1] migration: refactor migration_completion

[Wei Wang]

Current migration_completion function is a bit long. Refactor the long
implementation into different subfunctions:
- migration_completion_precopy: completion code related to precopy
- migration_completion_postcopy: completion code related to postcopy
- close_return_path_on_source: rp thread related cleanup on migration
completion. It is named to match with open_return_path_on_source.

This improves readability and is easier for future updates (e.g. add new
subfunctions when completion code related to new features are needed). No
functional changes intended.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
---
 migration/migration.c | 181 +++++++++++++++++++++++++-----------------
 1 file changed, 106 insertions(+), 75 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 91bba630a8..83f1c74f99 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2058,6 +2058,21 @@ static int await_return_path_close_on_source(MigrationState *ms)
     return ms->rp_state.error;
 }
 
+static int close_return_path_on_source(MigrationState *ms)
+{
+    int ret;
+
+    if (!ms->rp_state.rp_thread_created) {
+        return 0;
+    }
+
+    trace_migration_return_path_end_before();
+    ret = await_return_path_close_on_source(ms);
+    trace_migration_return_path_end_after(ret);
+
+    return ret;
+}
+
 static inline void
 migration_wait_main_channel(MigrationState *ms)
 {
@@ -2288,66 +2303,107 @@ static int migration_maybe_pause(MigrationState *s,
     return s->state == new_state ? 0 : -EINVAL;
 }
 
-/**
- * migration_completion: Used by migration_thread when there's not much left.
- *   The caller 'breaks' the loop when this returns.
- *
- * @s: Current migration state
- */
-static void migration_completion(MigrationState *s)
+static int migration_completion_precopy(MigrationState *s,
+                                        int *current_active_state)
 {
     int ret;
-    int current_active_state = s->state;
 
-    if (s->state == MIGRATION_STATUS_ACTIVE) {
-        qemu_mutex_lock_iothread();
-        s->downtime_start = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
-        qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER, NULL);
+    qemu_mutex_lock_iothread();
+    s->downtime_start = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
+    qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER, NULL);
 
-        s->vm_old_state = runstate_get();
-        global_state_store();
+    s->vm_old_state = runstate_get();
+    global_state_store();
 
-        ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
-        trace_migration_completion_vm_stop(ret);
-        if (ret >= 0) {
-            ret = migration_maybe_pause(s, &current_active_state,
-                                        MIGRATION_STATUS_DEVICE);
-        }
-        if (ret >= 0) {
-            /*
-             * Inactivate disks except in COLO, and track that we
-             * have done so in order to remember to reactivate
-             * them if migration fails or is cancelled.
-             */
-            s->block_inactive = !migrate_colo();
-            migration_rate_set(RATE_LIMIT_DISABLED);
-            ret = qemu_savevm_state_complete_precopy(s->to_dst_file, false,
-                                                     s->block_inactive);
-        }
+    ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
+    trace_migration_completion_vm_stop(ret);
+    if (ret < 0) {
+        goto out_unlock;
+    }
 
-        qemu_mutex_unlock_iothread();
+    ret = migration_maybe_pause(s, current_active_state,
+                                MIGRATION_STATUS_DEVICE);
+    if (ret < 0) {
+        goto out_unlock;
+    }
 
-        if (ret < 0) {
-            goto fail;
-        }
-    } else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
-        trace_migration_completion_postcopy_end();
+    /*
+     * Inactivate disks except in COLO, and track that we have done so in order
+     * to remember to reactivate them if migration fails or is cancelled.
+     */
+    s->block_inactive = !migrate_colo();
+    migration_rate_set(RATE_LIMIT_DISABLED);
+    ret = qemu_savevm_state_complete_precopy(s->to_dst_file, false,
+                                             s->block_inactive);
+out_unlock:
+    qemu_mutex_unlock_iothread();
+    return ret;
+}
 
-        qemu_mutex_lock_iothread();
-        qemu_savevm_state_complete_postcopy(s->to_dst_file);
-        qemu_mutex_unlock_iothread();
+static int migration_completion_postcopy(MigrationState *s)
+{
+    trace_migration_completion_postcopy_end();
+
+    qemu_mutex_lock_iothread();
+    qemu_savevm_state_complete_postcopy(s->to_dst_file);
+    qemu_mutex_unlock_iothread();
+
+    /*
+     * Shutdown the postcopy fast path thread.  This is only needed when dest
+     * QEMU binary is old (7.1/7.2).  QEMU 8.0+ doesn't need this.
+     */
+    if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
+        postcopy_preempt_shutdown_file(s);
+    }
+
+    trace_migration_completion_postcopy_end_after_complete();
+
+    return 0;
+}
 
+static void migration_completion_failed(MigrationState *s,
+                                        int current_active_state)
+{
+    if (s->block_inactive && (s->state == MIGRATION_STATUS_ACTIVE ||
+                              s->state == MIGRATION_STATUS_DEVICE)) {
         /*
-         * Shutdown the postcopy fast path thread.  This is only needed
-         * when dest QEMU binary is old (7.1/7.2).  QEMU 8.0+ doesn't need
-         * this.
+         * If not doing postcopy, vm_start() will be called: let's
+         * regain control on images.
          */
-        if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
-            postcopy_preempt_shutdown_file(s);
+        Error *local_err = NULL;
+
+        qemu_mutex_lock_iothread();
+        bdrv_activate_all(&local_err);
+        if (local_err) {
+            error_report_err(local_err);
+        } else {
+            s->block_inactive = false;
         }
+        qemu_mutex_unlock_iothread();
+    }
 
-        trace_migration_completion_postcopy_end_after_complete();
-    } else {
+    migrate_set_state(&s->state, current_active_state,
+                      MIGRATION_STATUS_FAILED);
+}
+
+/**
+ * migration_completion: Used by migration_thread when there's not much left.
+ *   The caller 'breaks' the loop when this returns.
+ *
+ * @s: Current migration state
+ */
+static void migration_completion(MigrationState *s)
+{
+    int ret = -1;
+    int current_active_state = s->state;
+
+    if (s->state == MIGRATION_STATUS_ACTIVE) {
+        ret = migration_completion_precopy(s, &current_active_state);
+    } else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
+        ret = migration_completion_postcopy(s);
+    }
+
+    if (ret < 0) {
         goto fail;
     }
 
@@ -2357,14 +2413,8 @@ static void migration_completion(MigrationState *s)
      * it will wait for the destination to send it's status in
      * a SHUT command).
      */
-    if (s->rp_state.rp_thread_created) {
-        int rp_error;
-        trace_migration_return_path_end_before();
-        rp_error = await_return_path_close_on_source(s);
-        trace_migration_return_path_end_after(rp_error);
-        if (rp_error) {
-            goto fail;
-        }
+    if (close_return_path_on_source(s) < 0) {
+        goto fail;
     }
 
     if (qemu_file_get_error(s->to_dst_file)) {
@@ -2384,26 +2434,7 @@ static void migration_completion(MigrationState *s)
     return;
 
 fail:
-    if (s->block_inactive && (s->state == MIGRATION_STATUS_ACTIVE ||
-                              s->state == MIGRATION_STATUS_DEVICE)) {
-        /*
-         * If not doing postcopy, vm_start() will be called: let's
-         * regain control on images.
-         */
-        Error *local_err = NULL;
-
-        qemu_mutex_lock_iothread();
-        bdrv_activate_all(&local_err);
-        if (local_err) {
-            error_report_err(local_err);
-        } else {
-            s->block_inactive = false;
-        }
-        qemu_mutex_unlock_iothread();
-    }
-
-    migrate_set_state(&s->state, current_active_state,
-                      MIGRATION_STATUS_FAILED);
+    migration_completion_failed(s, current_active_state);
 }
 
 /**
-- 
2.27.0

Re: [PATCH v1] migration: refactor migration_completion

[Isaku Yamahata]

On Fri, Jul 14, 2023 at 08:48:23PM +0800,
Wei Wang <wei.w.wang@intel.com> wrote:

> Current migration_completion function is a bit long. Refactor the long
> implementation into different subfunctions:
> - migration_completion_precopy: completion code related to precopy
> - migration_completion_postcopy: completion code related to postcopy
> - close_return_path_on_source: rp thread related cleanup on migration
> completion. It is named to match with open_return_path_on_source.
> 
> This improves readability and is easier for future updates (e.g. add new
> subfunctions when completion code related to new features are needed). No
> functional changes intended.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> ---
>  migration/migration.c | 181 +++++++++++++++++++++++++-----------------
>  1 file changed, 106 insertions(+), 75 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 91bba630a8..83f1c74f99 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2058,6 +2058,21 @@ static int await_return_path_close_on_source(MigrationState *ms)
>      return ms->rp_state.error;
>  }
>  
> +static int close_return_path_on_source(MigrationState *ms)
> +{
> +    int ret;
> +
> +    if (!ms->rp_state.rp_thread_created) {
> +        return 0;
> +    }
> +
> +    trace_migration_return_path_end_before();
> +    ret = await_return_path_close_on_source(ms);
> +    trace_migration_return_path_end_after(ret);
> +
> +    return ret;
> +}
> +

There is only one caller, migration_completion().  We can consolidate
two functions, await_return_path_close_on_source() and
close_return_path_on_source(), into single function.


>  static inline void
>  migration_wait_main_channel(MigrationState *ms)
>  {
> @@ -2288,66 +2303,107 @@ static int migration_maybe_pause(MigrationState *s,
>      return s->state == new_state ? 0 : -EINVAL;
>  }
>  
> -/**
> - * migration_completion: Used by migration_thread when there's not much left.
> - *   The caller 'breaks' the loop when this returns.
> - *
> - * @s: Current migration state
> - */
> -static void migration_completion(MigrationState *s)
> +static int migration_completion_precopy(MigrationState *s,
> +                                        int *current_active_state)
>  {
>      int ret;
> -    int current_active_state = s->state;
>  
> -    if (s->state == MIGRATION_STATUS_ACTIVE) {
> -        qemu_mutex_lock_iothread();
> -        s->downtime_start = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> -        qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER, NULL);
> +    qemu_mutex_lock_iothread();
> +    s->downtime_start = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> +    qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER, NULL);
>  
> -        s->vm_old_state = runstate_get();
> -        global_state_store();
> +    s->vm_old_state = runstate_get();
> +    global_state_store();
>  
> -        ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
> -        trace_migration_completion_vm_stop(ret);
> -        if (ret >= 0) {
> -            ret = migration_maybe_pause(s, &current_active_state,
> -                                        MIGRATION_STATUS_DEVICE);
> -        }
> -        if (ret >= 0) {
> -            /*
> -             * Inactivate disks except in COLO, and track that we
> -             * have done so in order to remember to reactivate
> -             * them if migration fails or is cancelled.
> -             */
> -            s->block_inactive = !migrate_colo();
> -            migration_rate_set(RATE_LIMIT_DISABLED);
> -            ret = qemu_savevm_state_complete_precopy(s->to_dst_file, false,
> -                                                     s->block_inactive);
> -        }
> +    ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
> +    trace_migration_completion_vm_stop(ret);
> +    if (ret < 0) {
> +        goto out_unlock;
> +    }
>  
> -        qemu_mutex_unlock_iothread();
> +    ret = migration_maybe_pause(s, current_active_state,
> +                                MIGRATION_STATUS_DEVICE);
> +    if (ret < 0) {
> +        goto out_unlock;
> +    }
>  
> -        if (ret < 0) {
> -            goto fail;
> -        }
> -    } else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> -        trace_migration_completion_postcopy_end();
> +    /*
> +     * Inactivate disks except in COLO, and track that we have done so in order
> +     * to remember to reactivate them if migration fails or is cancelled.
> +     */
> +    s->block_inactive = !migrate_colo();
> +    migration_rate_set(RATE_LIMIT_DISABLED);
> +    ret = qemu_savevm_state_complete_precopy(s->to_dst_file, false,
> +                                             s->block_inactive);
> +out_unlock:
> +    qemu_mutex_unlock_iothread();
> +    return ret;
> +}
>  
> -        qemu_mutex_lock_iothread();
> -        qemu_savevm_state_complete_postcopy(s->to_dst_file);
> -        qemu_mutex_unlock_iothread();
> +static int migration_completion_postcopy(MigrationState *s)
> +{
> +    trace_migration_completion_postcopy_end();
> +
> +    qemu_mutex_lock_iothread();
> +    qemu_savevm_state_complete_postcopy(s->to_dst_file);
> +    qemu_mutex_unlock_iothread();
> +
> +    /*
> +     * Shutdown the postcopy fast path thread.  This is only needed when dest
> +     * QEMU binary is old (7.1/7.2).  QEMU 8.0+ doesn't need this.
> +     */
> +    if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
> +        postcopy_preempt_shutdown_file(s);
> +    }
> +
> +    trace_migration_completion_postcopy_end_after_complete();
> +
> +    return 0;

Always return 0?  Make it void.


> +}
>  
> +static void migration_completion_failed(MigrationState *s,
> +                                        int current_active_state)
> +{
> +    if (s->block_inactive && (s->state == MIGRATION_STATUS_ACTIVE ||
> +                              s->state == MIGRATION_STATUS_DEVICE)) {
>          /*
> -         * Shutdown the postcopy fast path thread.  This is only needed
> -         * when dest QEMU binary is old (7.1/7.2).  QEMU 8.0+ doesn't need
> -         * this.
> +         * If not doing postcopy, vm_start() will be called: let's
> +         * regain control on images.
>           */
> -        if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
> -            postcopy_preempt_shutdown_file(s);
> +        Error *local_err = NULL;
> +
> +        qemu_mutex_lock_iothread();
> +        bdrv_activate_all(&local_err);
> +        if (local_err) {
> +            error_report_err(local_err);
> +        } else {
> +            s->block_inactive = false;
>          }
> +        qemu_mutex_unlock_iothread();
> +    }
>  
> -        trace_migration_completion_postcopy_end_after_complete();
> -    } else {
> +    migrate_set_state(&s->state, current_active_state,
> +                      MIGRATION_STATUS_FAILED);
> +}
> +
> +/**
> + * migration_completion: Used by migration_thread when there's not much left.
> + *   The caller 'breaks' the loop when this returns.
> + *
> + * @s: Current migration state
> + */
> +static void migration_completion(MigrationState *s)
> +{
> +    int ret = -1;
> +    int current_active_state = s->state;
> +
> +    if (s->state == MIGRATION_STATUS_ACTIVE) {
> +        ret = migration_completion_precopy(s, &current_active_state);
> +    } else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> +        ret = migration_completion_postcopy(s);

Here we can set ret = 0.


> +    }
> +
> +    if (ret < 0) {
>          goto fail;
>      }
>  
> @@ -2357,14 +2413,8 @@ static void migration_completion(MigrationState *s)
>       * it will wait for the destination to send it's status in
>       * a SHUT command).
>       */
> -    if (s->rp_state.rp_thread_created) {
> -        int rp_error;
> -        trace_migration_return_path_end_before();
> -        rp_error = await_return_path_close_on_source(s);
> -        trace_migration_return_path_end_after(rp_error);
> -        if (rp_error) {
> -            goto fail;
> -        }
> +    if (close_return_path_on_source(s) < 0) {
> +        goto fail;
>      }
>  
>      if (qemu_file_get_error(s->to_dst_file)) {
> @@ -2384,26 +2434,7 @@ static void migration_completion(MigrationState *s)
>      return;
>  
>  fail:
> -    if (s->block_inactive && (s->state == MIGRATION_STATUS_ACTIVE ||
> -                              s->state == MIGRATION_STATUS_DEVICE)) {
> -        /*
> -         * If not doing postcopy, vm_start() will be called: let's
> -         * regain control on images.
> -         */
> -        Error *local_err = NULL;
> -
> -        qemu_mutex_lock_iothread();
> -        bdrv_activate_all(&local_err);
> -        if (local_err) {
> -            error_report_err(local_err);
> -        } else {
> -            s->block_inactive = false;
> -        }
> -        qemu_mutex_unlock_iothread();
> -    }
> -
> -    migrate_set_state(&s->state, current_active_state,
> -                      MIGRATION_STATUS_FAILED);
> +    migration_completion_failed(s, current_active_state);
>  }
>  
>  /**
> -- 
> 2.27.0
> 
> 

-- 
Isaku Yamahata <isaku.yamahata@gmail.com>

RE: [PATCH v1] migration: refactor migration_completion

[Wang, Wei W]

On Tuesday, July 18, 2023 1:44 PM, Isaku Yamahata wrote:
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -2058,6 +2058,21 @@ static int
> await_return_path_close_on_source(MigrationState *ms)
> >      return ms->rp_state.error;
> >  }
> >
> > +static int close_return_path_on_source(MigrationState *ms) {
> > +    int ret;
> > +
> > +    if (!ms->rp_state.rp_thread_created) {
> > +        return 0;
> > +    }
> > +
> > +    trace_migration_return_path_end_before();
> > +    ret = await_return_path_close_on_source(ms);
> > +    trace_migration_return_path_end_after(ret);
> > +
> > +    return ret;
> > +}
> > +
> 
> There is only one caller, migration_completion().  We can consolidate two
> functions, await_return_path_close_on_source() and
> close_return_path_on_source(), into single function.

Sounds good, thanks.

> > +static int migration_completion_postcopy(MigrationState *s) {
> > +    trace_migration_completion_postcopy_end();
> > +
> > +    qemu_mutex_lock_iothread();
> > +    qemu_savevm_state_complete_postcopy(s->to_dst_file);
> > +    qemu_mutex_unlock_iothread();
> > +
> > +    /*
> > +     * Shutdown the postcopy fast path thread.  This is only needed when
> dest
> > +     * QEMU binary is old (7.1/7.2).  QEMU 8.0+ doesn't need this.
> > +     */
> > +    if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
> > +        postcopy_preempt_shutdown_file(s);
> > +    }
> > +
> > +    trace_migration_completion_postcopy_end_after_complete();
> > +
> > +    return 0;
> 
> Always return 0?  Make it void.

OK.

> > +static void migration_completion(MigrationState *s) {
> > +    int ret = -1;
> > +    int current_active_state = s->state;
> > +
> > +    if (s->state == MIGRATION_STATUS_ACTIVE) {
> > +        ret = migration_completion_precopy(s, &current_active_state);
> > +    } else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> > +        ret = migration_completion_postcopy(s);
> 
> Here we can set ret = 0.

Yes, after migration_completion_postcopy is made "void".

Re: [PATCH v1] migration: refactor migration_completion

[Peter Xu]

On Tue, Jul 18, 2023 at 01:25:12PM +0000, Wang, Wei W wrote:
> On Tuesday, July 18, 2023 1:44 PM, Isaku Yamahata wrote:
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -2058,6 +2058,21 @@ static int
> > await_return_path_close_on_source(MigrationState *ms)
> > >      return ms->rp_state.error;
> > >  }
> > >
> > > +static int close_return_path_on_source(MigrationState *ms) {
> > > +    int ret;
> > > +
> > > +    if (!ms->rp_state.rp_thread_created) {
> > > +        return 0;
> > > +    }
> > > +
> > > +    trace_migration_return_path_end_before();
> > > +    ret = await_return_path_close_on_source(ms);
> > > +    trace_migration_return_path_end_after(ret);
> > > +
> > > +    return ret;
> > > +}
> > > +
> > 
> > There is only one caller, migration_completion().  We can consolidate two
> > functions, await_return_path_close_on_source() and
> > close_return_path_on_source(), into single function.
> 
> Sounds good, thanks.
> 
> > > +static int migration_completion_postcopy(MigrationState *s) {
> > > +    trace_migration_completion_postcopy_end();
> > > +
> > > +    qemu_mutex_lock_iothread();
> > > +    qemu_savevm_state_complete_postcopy(s->to_dst_file);
> > > +    qemu_mutex_unlock_iothread();
> > > +
> > > +    /*
> > > +     * Shutdown the postcopy fast path thread.  This is only needed when
> > dest
> > > +     * QEMU binary is old (7.1/7.2).  QEMU 8.0+ doesn't need this.
> > > +     */
> > > +    if (migrate_postcopy_preempt() && s->preempt_pre_7_2) {
> > > +        postcopy_preempt_shutdown_file(s);
> > > +    }
> > > +
> > > +    trace_migration_completion_postcopy_end_after_complete();
> > > +
> > > +    return 0;
> > 
> > Always return 0?  Make it void.
> 
> OK.
> 
> > > +static void migration_completion(MigrationState *s) {
> > > +    int ret = -1;
> > > +    int current_active_state = s->state;
> > > +
> > > +    if (s->state == MIGRATION_STATUS_ACTIVE) {
> > > +        ret = migration_completion_precopy(s, &current_active_state);
> > > +    } else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> > > +        ret = migration_completion_postcopy(s);
> > 
> > Here we can set ret = 0.
> 
> Yes, after migration_completion_postcopy is made "void".

Looks good to me, after addressing Isaku's comments.

The current_active_state is very unfortunate, along with most of the calls
to migrate_set_state() - I bet most of the code will definitely go wrong if
that cmpxchg didn't succeed inside of migrate_set_state(), IOW in most
cases we simply always want:

  migrate_set_state(&s->state, s->state, XXX);

Not sure whether one pre-requisite patch is good to have so we can rename
migrate_set_state() to something like __migrate_set_state(), then:

  migrate_set_state(s, XXX) {
    __migrate_set_state(&s->state, s->state, XXX);
  }

I don't even know whether there's any call site that will need
__migrate_set_state() for real..

Thanks,

-- 
Peter Xu

RE: [PATCH v1] migration: refactor migration_completion

[Wang, Wei W]

On Friday, July 21, 2023 4:38 AM, Peter Xu wrote:
> Looks good to me, after addressing Isaku's comments.
> 
> The current_active_state is very unfortunate, along with most of the calls to
> migrate_set_state() - I bet most of the code will definitely go wrong if that
> cmpxchg didn't succeed inside of migrate_set_state(), IOW in most cases we
> simply always want:

Can you share examples where it could be wrong?
(If it has bugs, we need to fix)

> 
>   migrate_set_state(&s->state, s->state, XXX);
> 
> Not sure whether one pre-requisite patch is good to have so we can rename
> migrate_set_state() to something like __migrate_set_state(), then:
> 
>   migrate_set_state(s, XXX) {
>     __migrate_set_state(&s->state, s->state, XXX);
>   }
> 
> I don't even know whether there's any call site that will need
> __migrate_set_state() for real..
> 

Seems this would break the use of "MIGRATION_STATUS_CANCELLING".
For example, 
- In migration_maybe_pause:
migrate_set_state(&s->state, MIGRATION_STATUS_PRE_SWITCHOVER,
                                    new_state);
If the current s->state isn't MIGRATION_STATUS_PRE_SWITCHOVER (could
be MIGRATION_STATUS_CANCELLING),  then s->state won’t be updated to
new_state.
- Then, in migration_completion, the following update to s->state won't succeed:
   migrate_set_state(&s->state, current_active_state,
                          MIGRATION_STATUS_COMPLETED);

- Finally, when reaching migration_iteration_finish(), s->state is
MIGRATION_STATUS_CANCELLING, instead of MIGRATION_STATUS_COMPLETED.

Re: [PATCH v1] migration: refactor migration_completion

[Peter Xu]

On Fri, Jul 21, 2023 at 11:14:55AM +0000, Wang, Wei W wrote:
> On Friday, July 21, 2023 4:38 AM, Peter Xu wrote:
> > Looks good to me, after addressing Isaku's comments.
> > 
> > The current_active_state is very unfortunate, along with most of the calls to
> > migrate_set_state() - I bet most of the code will definitely go wrong if that
> > cmpxchg didn't succeed inside of migrate_set_state(), IOW in most cases we
> > simply always want:
> 
> Can you share examples where it could be wrong?
> (If it has bugs, we need to fix)

Nop.  What I meant is most of the cases we want to set the state without
caring much about the old state, so at least we can have a helper like
below and simply call migrate_set_state(s, STATE) where we don't care old
state.

> 
> > 
> >   migrate_set_state(&s->state, s->state, XXX);
> > 
> > Not sure whether one pre-requisite patch is good to have so we can rename
> > migrate_set_state() to something like __migrate_set_state(), then:
> > 
> >   migrate_set_state(s, XXX) {
> >     __migrate_set_state(&s->state, s->state, XXX);
> >   }
> > 
> > I don't even know whether there's any call site that will need
> > __migrate_set_state() for real..
> > 
> 
> Seems this would break the use of "MIGRATION_STATUS_CANCELLING".
> For example, 
> - In migration_maybe_pause:
> migrate_set_state(&s->state, MIGRATION_STATUS_PRE_SWITCHOVER,
>                                     new_state);
> If the current s->state isn't MIGRATION_STATUS_PRE_SWITCHOVER (could
> be MIGRATION_STATUS_CANCELLING),  then s->state won’t be updated to
> new_state.
> - Then, in migration_completion, the following update to s->state won't succeed:
>    migrate_set_state(&s->state, current_active_state,
>                           MIGRATION_STATUS_COMPLETED);
> 
> - Finally, when reaching migration_iteration_finish(), s->state is
> MIGRATION_STATUS_CANCELLING, instead of MIGRATION_STATUS_COMPLETED.

The whole state changes are just flaky to me in general, even with the help
of old_state cmpxchg.

E.g., I'm wondering whether below race can happen, assuming we're starting
with ACTIVE state and just about to complete migration:

          main thread                            migration thread
          -----------                            ----------------
                                           migration_maybe_pause(current_active_state==ACTIVE)
                                             if (s->state != MIGRATION_STATUS_CANCELLING)
                                               --> true, keep setting state
                                               qemu_mutex_unlock_iothread();
    qemu_mutex_lock_iothread();
    migrate_fd_cancel()
      if (old_state == MIGRATION_STATUS_PRE_SWITCHOVER)
        --> false, not posting to pause_sem
      set state to MIGRATION_STATUS_CANCELLING
                                              migrate_set_state(&s->state, *current_active_state,
                                                                MIGRATION_STATUS_PRE_SWITCHOVER);
                                                --> false, cmpxchg fail
                                              qemu_sem_wait(&s->pause_sem);
                                                --> hang death?

Thanks,

-- 
Peter Xu

RE: [PATCH v1] migration: refactor migration_completion

[Wang, Wei W]

On Thursday, July 27, 2023 1:10 AM, Peter Xu wrote:
> On Fri, Jul 21, 2023 at 11:14:55AM +0000, Wang, Wei W wrote:
> > On Friday, July 21, 2023 4:38 AM, Peter Xu wrote:
> > > Looks good to me, after addressing Isaku's comments.
> > >
> > > The current_active_state is very unfortunate, along with most of the
> > > calls to
> > > migrate_set_state() - I bet most of the code will definitely go
> > > wrong if that cmpxchg didn't succeed inside of migrate_set_state(),
> > > IOW in most cases we simply always want:
> >
> > Can you share examples where it could be wrong?
> > (If it has bugs, we need to fix)
> 
> Nop.  What I meant is most of the cases we want to set the state without
> caring much about the old state, so at least we can have a helper like below
> and simply call migrate_set_state(s, STATE) where we don't care old state.
> 
> >
> > >
> > >   migrate_set_state(&s->state, s->state, XXX);
> > >
> > > Not sure whether one pre-requisite patch is good to have so we can
> > > rename
> > > migrate_set_state() to something like __migrate_set_state(), then:
> > >
> > >   migrate_set_state(s, XXX) {
> > >     __migrate_set_state(&s->state, s->state, XXX);
> > >   }
> > >
> > > I don't even know whether there's any call site that will need
> > > __migrate_set_state() for real..
> > >
> >
> > Seems this would break the use of "MIGRATION_STATUS_CANCELLING".
> > For example,
> > - In migration_maybe_pause:
> > migrate_set_state(&s->state, MIGRATION_STATUS_PRE_SWITCHOVER,
> >                                     new_state); If the current
> > s->state isn't MIGRATION_STATUS_PRE_SWITCHOVER (could be
> > MIGRATION_STATUS_CANCELLING),  then s->state won’t be updated to
> > new_state.
> > - Then, in migration_completion, the following update to s->state won't
> succeed:
> >    migrate_set_state(&s->state, current_active_state,
> >                           MIGRATION_STATUS_COMPLETED);
> >
> > - Finally, when reaching migration_iteration_finish(), s->state is
> > MIGRATION_STATUS_CANCELLING, instead of
> MIGRATION_STATUS_COMPLETED.
> 
> The whole state changes are just flaky to me in general, even with the help of
> old_state cmpxchg.

Yes, the design/implementation of the migration state transition can be
improved (it looks fragile to me). I think this should be done in a separate
patchset, though. For this patch, we could keep it no functional change.

> 
> E.g., I'm wondering whether below race can happen, assuming we're starting
> with ACTIVE state and just about to complete migration:
> 
>           main thread                            migration thread
>           -----------                            ----------------
> 
> migration_maybe_pause(current_active_state==ACTIVE)
>                                              if (s->state != MIGRATION_STATUS_CANCELLING)
>                                                --> true, keep setting state
>                                                qemu_mutex_unlock_iothread();
>     qemu_mutex_lock_iothread();
>     migrate_fd_cancel()
>       if (old_state == MIGRATION_STATUS_PRE_SWITCHOVER)
>         --> false, not posting to pause_sem
>       set state to MIGRATION_STATUS_CANCELLING
>                                               migrate_set_state(&s->state, *current_active_state,
>                                                                 MIGRATION_STATUS_PRE_SWITCHOVER);
>                                                 --> false, cmpxchg fail
>                                               qemu_sem_wait(&s->pause_sem);
>                                                 --> hang death?

Still need "migrate continue" to unblock the migration thread.
Probably we should document that PAUSE_BEFORE_SWITCHOVER always requires an
explicit "migrate continue" to be issued from user (even after migration is cancelled).