Re: [PATCH v7 07/12] multifd: Prepare to send a packet without the mutex held

[Leonardo Brás]

On Tue, 2022-08-02 at 08:39 +0200, Juan Quintela wrote:
> We do the send_prepare() and the fill of the head packet without the
> mutex held.  It will help a lot for compression and later in the
> series for zero pages.
> 
> Notice that we can use p->pages without holding p->mutex because
> p->pending_job == 1.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/multifd.h |  2 ++
>  migration/multifd.c | 11 ++++++-----
>  2 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/migration/multifd.h b/migration/multifd.h
> index a67cefc0a2..cd389d18d2 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -109,7 +109,9 @@ typedef struct {
>      /* array of pages to sent.
>       * The owner of 'pages' depends of 'pending_job' value:
>       * pending_job == 0 -> migration_thread can use it.
> +     *                     No need for mutex lock.
>       * pending_job != 0 -> multifd_channel can use it.
> +     *                     No need for mutex lock.
>       */
>      MultiFDPages_t *pages;
>  
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 09a40a9135..68fc9f8e88 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -663,6 +663,8 @@ static void *multifd_send_thread(void *opaque)
>                  p->flags |= MULTIFD_FLAG_SYNC;
>                  p->sync_needed = false;
>              }
> +            qemu_mutex_unlock(&p->mutex);
> +

If it unlocks here, we will have unprotected:
for (int i = 0; i < p->pages->num; i++) {
    p->normal[p->normal_num] = p->pages->offset[i];
    p->normal_num++;
}

And p->pages seems to be in the mutex-protected area.
Should it be ok?

Also, under that we have:
            if (p->normal_num) {
                ret = multifd_send_state->ops->send_prepare(p, &local_err);
                if (ret != 0) {
                    qemu_mutex_unlock(&p->mutex);
                    break;
                }
            }

Calling mutex_unlock() here, even though the unlock already happened before,
could cause any issue?


>              p->normal_num = 0;
>  
>              if (use_zero_copy_send) {
> @@ -684,11 +686,6 @@ static void *multifd_send_thread(void *opaque)
>                  }
>              }
>              multifd_send_fill_packet(p);
> -            p->num_packets++;
> -            p->total_normal_pages += p->normal_num;
> -            p->pages->num = 0;
> -            p->pages->block = NULL;
> -            qemu_mutex_unlock(&p->mutex);
>  
>              trace_multifd_send(p->id, packet_num, p->normal_num, p->flags,
>                                 p->next_packet_size);
> @@ -713,6 +710,10 @@ static void *multifd_send_thread(void *opaque)
>              }
>  
>              qemu_mutex_lock(&p->mutex);
> +            p->num_packets++;
> +            p->total_normal_pages += p->normal_num;
> +            p->pages->num = 0;
> +            p->pages->block = NULL;
>              p->sent_bytes += p->packet_len;;
>              p->sent_bytes += p->next_packet_size;
>              p->pending_job--;

Not used in the interval, this part seems ok.

Best regards,
Leo