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

[Peter Xu]

On Fri, Mar 31, 2017 at 04:25:56PM +0100, Dr. David Alan Gilbert wrote:
> * 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.

Thanks for the confirmation.

(ps: even if we are not with bql, we should not need this
 rcu_read_lock, right? My understanding is: if we want to protect
 src_page_requests, we should need the mutex, not rcu lock; while for
 the memory_region_unref() since we have had the reference, looks like
 we don't need any kind of locking either)

> 
> 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

-- peterx