Re: [PATCH RFC 3/7] migration: multifd_send_kick_main()

[Peter Xu]

On Wed, Nov 08, 2023 at 07:49:53PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > When a multifd sender thread hit errors, it always needs to kick the main
> > thread by kicking all the semaphores that it can be waiting upon.
> >
> > Provide a helper for it and deduplicate the code.
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/multifd.c | 21 +++++++++++++++------
> >  1 file changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/migration/multifd.c b/migration/multifd.c
> > index 4afdd88602..33fb21d0e4 100644
> > --- a/migration/multifd.c
> > +++ b/migration/multifd.c
> > @@ -374,6 +374,18 @@ struct {
> >      MultiFDMethods *ops;
> >  } *multifd_send_state;
> >  
> > +/*
> > + * The migration thread can wait on either of the two semaphores.  This
> > + * function can be used to kick the main thread out of waiting on either of
> > + * them.  Should mostly only be called when something wrong happened with
> > + * the current multifd send thread.
> > + */
> > +static void multifd_send_kick_main(MultiFDSendParams *p)
> > +{
> > +    qemu_sem_post(&p->sem_sync);
> > +    qemu_sem_post(&multifd_send_state->channels_ready);
> > +}
> > +
> >  /*
> >   * How we use multifd_send_state->pages and channel->pages?
> >   *
> > @@ -746,8 +758,7 @@ out:
> >          assert(local_err);
> >          trace_multifd_send_error(p->id);
> >          multifd_send_terminate_threads(local_err);
> > -        qemu_sem_post(&p->sem_sync);
> > -        qemu_sem_post(&multifd_send_state->channels_ready);
> > +        multifd_send_kick_main(p);
> >          error_free(local_err);
> >      }
> >  
> > @@ -787,8 +798,7 @@ static void multifd_tls_outgoing_handshake(QIOTask 
> > *task,
> >       * is not created, and then tell who pay attention to me.
> >       */
> >      p->quit = true;
> > -    qemu_sem_post(&multifd_send_state->channels_ready);
> > -    qemu_sem_post(&p->sem_sync);
> > +    multifd_send_kick_main(p);
> 
> There's a bug here in the original code:
> 
> It's not really safe to call any of these outside of the channel lock
> because multifd_save_cleanup() could execute at the same time and call
> qemu_sem_destroy() -> qemu_mutex_destroy(), which can assert because we
> might be holding the sem_lock.

If you meant "p->mutex" as the "channel lock", IIUC even holding that won't
work? Because it'll also be freed in multifd_save_cleanup().



> 
> It seems the reason we get away with this today is merely due to
> timing. A subset of this problem was already encountered here:
> 
> [PATCH] migrate/multifd: fix coredump when the multifd thread cleanup
> https://lore.kernel.org/r/20230621081826.3203053-1-zhangjianguo18@huawei.com
> 
> We could probably release the semaphores for all channels at once inside
> multifd_save_cleanup() in the main thread. We'd have a
> multifd_send_kick_main() in each channel when it fails and this:
> 
> void multifd_save_cleanup(void)
> {
>     ...   
>     for (i = 0; i < migrate_multifd_channels(); i++) {
>         MultiFDSendParams *p = &multifd_send_state->params[i];
> 
>         if (p->running) {
>             qemu_thread_join(&p->thread);
>         } else {
>             multifd_send_kick_main(p);
>         }
>     }
>     for (i = 0; i < migrate_multifd_channels(); i++) {
>         qemu_sem_destroy, etc...
>     }
>     ...
> }    
> 

-- 
Peter Xu