Re: [PATCH 2/3] migration/multifd: Protect accesses to migration_threads

[Juan Quintela <quintela@redhat.com>]

Fabiano Rosas <farosas@suse.de> wrote:
> This doubly linked list is common for all the multifd and migration
> threads so we need to avoid concurrent access.
>
> Add a mutex to protect the data from concurrent access. This fixes a
> crash when removing two MigrationThread objects from the list at the
> same time during cleanup of multifd threads.
>
> To avoid destroying the mutex before the last element has been
> removed, move calls to qmp_migration_thread_remove so they run before
> multifd_save_cleanup joins the threads.
>
> Fixes: 671326201d ("migration: Introduce interface query-migrationthreads")
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

I agree with Peter here.  Why don't you have to protect the walking?

> ---
>  migration/migration.c  |  5 ++++-
>  migration/multifd.c    |  3 ++-
>  migration/threadinfo.c | 19 ++++++++++++++++++-
>  migration/threadinfo.h |  5 +++--
>  4 files changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/migration/migration.c b/migration/migration.c
> index e731fc98a1..b3b8345eb2 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1146,6 +1146,7 @@ static void migrate_fd_cleanup(MigrationState *s)
>          qemu_mutex_lock_iothread();
>  
>          multifd_save_cleanup();
> +        qmp_migration_threads_cleanup();

I think I will spare this one as the mutex is static, so we are not
winning any memory back.

>      }
>  
>      trace_migration_thread_after_loop();
> +    qmp_migration_threads_remove(thread);
>      migration_iteration_finish(s);

I can understand moving it here, but why before migration_iteration_finish?

>      object_unref(OBJECT(s));
>      rcu_unregister_thread();
> -    qmp_migration_threads_remove(thread);
>      return NULL;
>  }
> +    qmp_migration_threads_remove(thread);
> +
>      qemu_mutex_lock(&p->mutex);
>      p->running = false;
>      qemu_mutex_unlock(&p->mutex);
>  
>      rcu_unregister_thread();
> -    qmp_migration_threads_remove(thread);
>      trace_multifd_send_thread_end(p->id, p->num_packets, p->total_normal_pages);
>  
>      return NULL;

Here it looks like the right place.


> +#include "qemu/osdep.h"
> +#include "qemu/queue.h"
> +#include "qemu/lockable.h"
>  #include "threadinfo.h"

Ouch, it missed Markus cleanup.  Thanks.

For the rest it looks good.

Later, Juan.