Re: [Qemu-devel] [PATCH 3/7] migration: Wait for semaphore before completing migration

[Dr. David Alan Gilbert]

* Peter Xu (address@hidden) wrote:
> On Wed, Oct 11, 2017 at 08:13:13PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > Wait for a semaphore before completing the migration,
> > if the previously added capability was enabled.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >  migration/migration.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> >  migration/migration.h |  3 +++
> >  2 files changed, 50 insertions(+)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index e1a87c3d23..b411a7bb63 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -1967,6 +1967,46 @@ fail:
> >  }
> >  
> >  /**
> > + * migration_maybe_pause: Pause if required to by 
> > migrate_pause_before_device
> > + * called with the iothread locked
> > + * Returns: 0 on success
> > + */
> > +static int migration_maybe_pause(MigrationState *s, int 
> > *current_active_state)
> > +{
> > +    int ret;
> > +    if (!migrate_pause_before_device()) {
> > +        return 0;
> > +    }
> > +    ret = bdrv_inactivate_all();
> 
> My understanding is that the crash was caused by mirrored block device
> IO triggered after the inactivation, then... should we do this after
> waiting for the semaphore (possibly at [1] below) to make sure the
> block jobs are completed?  Or did I miss anything?

Ah you're right, just confirmed this with kwolf that I got it the
wrong way around.
I'll fix it.

Dave

> > +    if (ret) {
> > +        error_report("%s: bdrv_inactivate_all() failed (%d)",
> > +                     __func__, ret);
> > +        return ret;
> > +    }
> > +
> > +    s->block_inactive = true;
> > +
> > +    /* Since leaving this state is not atomic with posting the semaphore
> > +     * it's possible that someone could have issued multiple 
> > migrate_continue
> > +     * and the semaphore is incorrectly positive at this point;
> > +     * the docs say it's undefined to reinit a semaphore that's already
> > +     * init'd, so use timedwait to eat up any existing posts.
> > +     */
> > +    while (qemu_sem_timedwait(&s->pause_sem, 1) == 0);
> > +
> > +    qemu_mutex_unlock_iothread();
> > +    migrate_set_state(&s->state, *current_active_state,
> > +                      MIGRATION_STATUS_PAUSE_BEFORE_DEVICE);
> > +    qemu_sem_wait(&s->pause_sem);
> 
> [1]
> 
> > +    migrate_set_state(&s->state, MIGRATION_STATUS_PAUSE_BEFORE_DEVICE,
> > +                      MIGRATION_STATUS_DEVICE);
> > +    *current_active_state = MIGRATION_STATUS_DEVICE;
> > +    qemu_mutex_lock_iothread();
> > +
> > +    return s->state == MIGRATION_STATUS_DEVICE ? 0 : -EINVAL;
> > +}
> > +
> > +/**
> >   * migration_completion: Used by migration_thread when there's not much 
> > left.
> >   *   The caller 'breaks' the loop when this returns.
> >   *
> > @@ -1992,6 +2032,11 @@ static void migration_completion(MigrationState *s, 
> > int current_active_state,
> >              bool inactivate = !migrate_colo_enabled();
> >              ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
> >              if (ret >= 0) {
> > +                ret = migration_maybe_pause(s, &current_active_state);
> > +                /* If this worked it will already have inactivated */
> > +                inactivate &= !migrate_pause_before_device();
> > +            }
> > +            if (ret >= 0) {
> >                  qemu_file_set_rate_limit(s->to_dst_file, INT64_MAX);
> >                  ret = qemu_savevm_state_complete_precopy(s->to_dst_file, 
> > false,
> >                                                           inactivate);
> > @@ -2372,6 +2417,7 @@ static void migration_instance_finalize(Object *obj)
> >  
> >      g_free(params->tls_hostname);
> >      g_free(params->tls_creds);
> > +    qemu_sem_destroy(&ms->pause_sem);
> >  }
> >  
> >  static void migration_instance_init(Object *obj)
> > @@ -2382,6 +2428,7 @@ static void migration_instance_init(Object *obj)
> >      ms->state = MIGRATION_STATUS_NONE;
> >      ms->xbzrle_cache_size = DEFAULT_MIGRATE_CACHE_SIZE;
> >      ms->mbps = -1;
> > +    qemu_sem_init(&ms->pause_sem, 0);
> >  
> >      params->tls_hostname = g_strdup("");
> >      params->tls_creds = g_strdup("");
> > diff --git a/migration/migration.h b/migration/migration.h
> > index 37feea5453..447e8b3f79 100644
> > --- a/migration/migration.h
> > +++ b/migration/migration.h
> > @@ -121,6 +121,9 @@ struct MigrationState
> >      /* Flag set once the migration thread called bdrv_inactivate_all */
> >      bool block_inactive;
> >  
> > +    /* Migration is paused due to pause-before-device */
> > +    QemuSemaphore pause_sem;
> > +
> >      /* The semaphore is used to notify COLO thread that failover is 
> > finished */
> >      QemuSemaphore colo_exit_sem;
> >  
> > -- 
> > 2.13.6
> > 
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK