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

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> 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().
>

You're right, I just sent an RFC about this, please take a look.