Re: [PATCH 05/17] migration/multifd: Wait for multifd channels creation before proceeding

[Fabiano Rosas]

Avihai Horon <avihaih@nvidia.com> writes:

> Currently, multifd channels are created asynchronously without waiting
> for their creation -- migration simply proceeds and may wait in
> multifd_send_sync_main(), which is called by ram_save_setup(). This
> hides in it some race conditions which can cause an unexpected behavior
> if some channels creation fail.
>
> For example, the following scenario of multifd migration with two
> channels, where the first channel creation fails, will end in a
> segmentation fault (time advances from top to bottom):

Is this reproducible? Or just observable at least.

I acknowledge the situation you describe, but with multifd there's
usually an issue in cleanup paths. Let's make sure we flushed those out
before adding this new semaphore.

This is similar to an issue Peter was addressing where we missed calling
multifd_send_termiante_threads() in the multifd_channel_connect() path:

patch 4 in this
https://lore.kernel.org/r/20231022201211.452861-1-peterx@redhat.com

>
> Thread           | Code execution
> ------------------------------------------------------------------------
> Multifd 1        |
>                  | multifd_new_send_channel_async (errors and quits)
>                  |   multifd_new_send_channel_cleanup
>                  |
> Migration thread |
>                  | qemu_savevm_state_setup
>                  |   ram_save_setup
>                  |     multifd_send_sync_main
>                  |     (detects Multifd 1 error and quits)
>                  | [...]
>                  | migration_iteration_finish
>                  |   migrate_fd_cleanup_schedule
>                  |
> Main thread      |
>                  | migrate_fd_cleanup
>                  |   multifd_save_cleanup (destroys Multifd 2 resources)
>                  |
> Multifd 2        |
>                  | multifd_new_send_channel_async
>                  | (accesses destroyed resources, segfault)
>
> In another scenario, migration can hang indefinitely:
> 1. Main migration thread reaches multifd_send_sync_main() and waits on
>    the semaphores.
> 2. Then, all multifd channels creation fails, so they post the
>    semaphores and quit.
> 3. Main migration channel will not identify the error, proceed to send
>    pages and will hang.
>
> Fix it by waiting for all multifd channels to be created before
> proceeding with migration.
>
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> ---
>  migration/multifd.h   |  3 +++
>  migration/migration.c |  1 +
>  migration/multifd.c   | 34 +++++++++++++++++++++++++++++++---
>  migration/ram.c       |  7 +++++++
>  4 files changed, 42 insertions(+), 3 deletions(-)
>
> diff --git a/migration/multifd.h b/migration/multifd.h
> index 35d11f103c..87a64e0a87 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -23,6 +23,7 @@ void multifd_recv_new_channel(QIOChannel *ioc, Error 
> **errp);
>  void multifd_recv_sync_main(void);
>  int multifd_send_sync_main(void);
>  int multifd_queue_page(RAMBlock *block, ram_addr_t offset);
> +int multifd_send_channels_created(void);
>  
>  /* Multifd Compression flags */
>  #define MULTIFD_FLAG_SYNC (1 << 0)
> @@ -86,6 +87,8 @@ typedef struct {
>      /* multifd flags for sending ram */
>      int write_flags;
>  
> +    /* Syncs channel creation and migration thread */
> +    QemuSemaphore create_sem;
>      /* sem where to wait for more work */
>      QemuSemaphore sem;
>      /* syncs main thread and channels */
> diff --git a/migration/migration.c b/migration/migration.c
> index 9c769a1ecd..d81d96eaa5 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3621,6 +3621,7 @@ void migrate_fd_connect(MigrationState *s, Error 
> *error_in)
>          error_report_err(local_err);
>          migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
>                            MIGRATION_STATUS_FAILED);
> +        multifd_send_channels_created();
>          migrate_fd_cleanup(s);
>          return;
>      }
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 564e911b6c..f0c216f4f9 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -538,6 +538,7 @@ void multifd_save_cleanup(void)
>          multifd_send_channel_destroy(p->c);
>          p->c = NULL;
>          qemu_mutex_destroy(&p->mutex);
> +        qemu_sem_destroy(&p->create_sem);
>          qemu_sem_destroy(&p->sem);
>          qemu_sem_destroy(&p->sem_sync);
>          g_free(p->name);
> @@ -766,6 +767,29 @@ out:
>      return NULL;
>  }
>  
> +int multifd_send_channels_created(void)
> +{
> +    int ret = 0;
> +
> +    if (!migrate_multifd()) {
> +        return 0;
> +    }
> +
> +    for (int i = 0; i < migrate_multifd_channels(); i++) {
> +        MultiFDSendParams *p = &multifd_send_state->params[i];
> +
> +        qemu_sem_wait(&p->create_sem);
> +        WITH_QEMU_LOCK_GUARD(&p->mutex) {
> +            if (p->quit) {
> +                error_report("%s: channel %d has already quit", __func__, i);
> +                ret = -1;
> +            }
> +        }

There are races here when a channel fails at
multifd_send_initial_packet(). If p->quit can be set after post to
create_sem, then this function could always return true and we'd run
into a broken channel. Possibly even the same bug you're trying to fix.

I think that's one of the reasons we have channels_ready.

> +    }
> +
> +    return ret;
> +}
> +
>  static bool multifd_channel_connect(MultiFDSendParams *p,
>                                      QIOChannel *ioc,
>                                      Error **errp);
> @@ -794,6 +818,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
>      p->quit = true;
>      qemu_sem_post(&multifd_send_state->channels_ready);
>      qemu_sem_post(&p->sem_sync);
> +    qemu_sem_post(&p->create_sem);
>      error_free(err);
>  }
>  
> @@ -857,6 +882,7 @@ static bool multifd_channel_connect(MultiFDSendParams *p,
>      qemu_thread_create(&p->thread, p->name, multifd_send_thread, p,
>                         QEMU_THREAD_JOINABLE);
>      p->running = true;
> +    qemu_sem_post(&p->create_sem);
>      return true;
>  }
>  
> @@ -864,15 +890,16 @@ static void 
> multifd_new_send_channel_cleanup(MultiFDSendParams *p,
>                                               QIOChannel *ioc, Error *err)
>  {
>       migrate_set_error(migrate_get_current(), err);
> -     /* Error happen, we need to tell who pay attention to me */
> -     qemu_sem_post(&multifd_send_state->channels_ready);
> -     qemu_sem_post(&p->sem_sync);
>       /*
> +      * Error happen, we need to tell who pay attention to me.
>        * Although multifd_send_thread is not created, but main migration
>        * thread need to judge whether it is running, so we need to mark
>        * its status.
>        */
>       p->quit = true;
> +     qemu_sem_post(&multifd_send_state->channels_ready);
> +     qemu_sem_post(&p->sem_sync);
> +     qemu_sem_post(&p->create_sem);

Do we still need channels_ready and sem_sync here? The migration thread
shouldn't have gone past create_sem at this point.

>       object_unref(OBJECT(ioc));
>       error_free(err);
>  }
> @@ -921,6 +948,7 @@ int multifd_save_setup(Error **errp)
>          MultiFDSendParams *p = &multifd_send_state->params[i];
>  
>          qemu_mutex_init(&p->mutex);
> +        qemu_sem_init(&p->create_sem, 0);
>          qemu_sem_init(&p->sem, 0);
>          qemu_sem_init(&p->sem_sync, 0);
>          p->quit = false;
> diff --git a/migration/ram.c b/migration/ram.c
> index c0cdcccb75..b3e864a22b 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2937,6 +2937,13 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>      RAMBlock *block;
>      int ret;
>  
> +    bql_unlock();
> +    ret = multifd_send_channels_created();
> +    bql_lock();
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
>      if (compress_threads_save_setup()) {
>          return -1;
>      }