Re: [Qemu-devel] [PATCH 30/51] ram: Move src_page_req* to RAMState

[Dr. David Alan Gilbert]

* Peter Xu (address@hidden) wrote:
> On Thu, Mar 23, 2017 at 09:45:23PM +0100, Juan Quintela wrote:
> > This are the last postcopy fields still at MigrationState.  Once there
> 
> s/This/These/
> 
> > Move MigrationSrcPageRequest to ram.c and remove MigrationState
> > parameters where appropiate.
> > 
> > Signed-off-by: Juan Quintela <address@hidden>
> 
> Reviewed-by: Peter Xu <address@hidden>
> 
> One question below though...
> 
> [...]
> 
> > @@ -1191,19 +1204,18 @@ static bool get_queued_page(RAMState *rs, 
> > MigrationState *ms,
> >   *
> >   * It should be empty at the end anyway, but in error cases there may
> >   * xbe some left.
> > - *
> > - * @ms: current migration state
> >   */
> > -void flush_page_queue(MigrationState *ms)
> > +void flush_page_queue(void)
> >  {
> > -    struct MigrationSrcPageRequest *mspr, *next_mspr;
> > +    struct RAMSrcPageRequest *mspr, *next_mspr;
> > +    RAMState *rs = &ram_state;
> >      /* This queue generally should be empty - but in the case of a failed
> >       * migration might have some droppings in.
> >       */
> >      rcu_read_lock();
> 
> Could I ask why we are taking the RCU read lock rather than the mutex
> here?

It's a good question whether we need anything at all.
flush_page_queue is called only from migrate_fd_cleanup.
migrate_fd_cleanup is called either from a backhalf, which I think has the bql,
or from a failure path in migrate_fd_connect.
migrate_fd_connect is called from migration_channel_connect and 
rdma_start_outgoing_migration
which I think both end up at monitor commands so also in the bql.

So I think we can probably just lose the rcu_read_lock/unlock.

Dave

> 
> > -    QSIMPLEQ_FOREACH_SAFE(mspr, &ms->src_page_requests, next_req, 
> > next_mspr) {
> > +    QSIMPLEQ_FOREACH_SAFE(mspr, &rs->src_page_requests, next_req, 
> > next_mspr) {
> >          memory_region_unref(mspr->rb->mr);
> > -        QSIMPLEQ_REMOVE_HEAD(&ms->src_page_requests, next_req);
> > +        QSIMPLEQ_REMOVE_HEAD(&rs->src_page_requests, next_req);
> >          g_free(mspr);
> >      }
> >      rcu_read_unlock();
> 
> Thanks,
> 
> -- peterx
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK