Re: [PATCH v2 2/5] migration: Make from_dst_file accesses thread-safe

[Peter Xu <peterx@redhat.com>]

On Wed, Jul 21, 2021 at 04:15:27PM -0500, Eric Blake wrote:
> On Wed, Jul 21, 2021 at 03:34:06PM -0400, Peter Xu wrote:
> > Accessing from_dst_file is potentially racy in current code base like below:
> > 
> >   if (s->from_dst_file)
> >     do_something(s->from_dst_file);
> > 
> > Because from_dst_file can be reset right after the check in another
> > thread (rp_thread).  One example is migrate_fd_cancel().
> > 
> > Use the same qemu_file_lock to protect it too, just like to_dst_file.
> > 
> > When it's safe to access without lock, comment it.
> > 
> > There's one special reference in migration_thread() that can be replaced by
> > the newly introduced rp_thread_created flag.
> > 
> > Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/migration.c | 32 +++++++++++++++++++++++++-------
> >  migration/migration.h |  8 +++++---
> >  migration/ram.c       |  1 +
> >  3 files changed, 31 insertions(+), 10 deletions(-)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 21b94f75a3..fa70400f98 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -1879,10 +1879,12 @@ static void migrate_fd_cancel(MigrationState *s)
> >      QEMUFile *f = migrate_get_current()->to_dst_file;
> >      trace_migrate_fd_cancel();
> >  
> > +    qemu_mutex_lock(&s->qemu_file_lock);
> >      if (s->rp_state.from_dst_file) {
> >          /* shutdown the rp socket, so causing the rp thread to shutdown */
> >          qemu_file_shutdown(s->rp_state.from_dst_file);
> >      }
> > +    qemu_mutex_unlock(&s->qemu_file_lock);
> 
> Worth using WITH_QEMU_LOCK_GUARD?

Sure.

> 
> > @@ -2827,11 +2845,13 @@ out:
> >               * Maybe there is something we can do: it looks like a
> >               * network down issue, and we pause for a recovery.
> >               */
> > -            qemu_fclose(rp);
> > -            ms->rp_state.from_dst_file = NULL;
> > +            migration_release_from_dst_file(ms);
> >              rp = NULL;
> >              if (postcopy_pause_return_path_thread(ms)) {
> > -                /* Reload rp, reset the rest */
> > +                /*
> > +                 * Reload rp, reset the rest.  Referencing it is save since
> 
> s/save/safe/

Will fix.

I'll wait for some more comments before I repost.

Thanks,

-- 
Peter Xu