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

[Leonardo Bras Soares Passos]

On Fri, Aug 19, 2022 at 8:32 AM Juan Quintela <quintela@redhat.com> wrote:
>
> Leonardo Brás <leobras@redhat.com> wrote:
> > 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?
>
> From the documentation:
>
>     /* 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;
>
> So, it is right.

Oh, right. I missed that part earlier .

>
> > 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?
>
> Good catch.  Never got an error there.
>
> Removing that bit.

Thanks!

Best regards,
Leo

>
> > Best regards,
>
>
> Thanks, Juan.
>