Re: BUG at net/sunrpc/svc_xprt.c:921

[Stanislav Kinsbursky]

17.01.2013 17:03, J. Bruce Fields ÐÐÑÐÑ:
> On Thu, Jan 17, 2013 at 09:05:51AM +0400, Stanislav Kinsbursky wrote:
> > 17.01.2013 02:51, Mark Lord ÐÐÑÐÑ:
> > > On 13-01-16 12:20 AM, Stanislav Kinsbursky wrote:
> > > > Mark, could you provide any call traces?
> > >
> > > Call traces from where/what?
> > > There's this one, posted earlier in the BUG report:
> > >
> > > kernel BUG at net/sunrpc/svc_xprt.c:921!
> > > Call Trace:
> > >   [<ffffffffa016a56a>] ? svc_recv+0xcc/0x338 [sunrpc]
> > >   [<ffffffffa0318bfc>] ? nfs_callback_authenticate+0x20/0x20 [nfsv4]
> > >   [<ffffffffa0318c19>] ? nfs4_callback_svc+0x1d/0x3c [nfsv4]
> > >   [<ffffffff810407e6>] ? kthread+0x81/0x89
> > >   [<ffffffff81040765>] ? kthread_freezable_should_stop+0x36/0x36
> > >   [<ffffffff812ea62c>] ? ret_from_fork+0x7c/0xb0
> > >   [<ffffffff81040765>] ? kthread_freezable_should_stop+0x36/0x36
> >
> > Thanks!
> > I haven't seen the bug report.
> > Could you provide the link, please?
>
> There's no bz if that's what you're asking for.
>
> See the first message in the thread for the original report:
>
> 	http://mid.gmane.org/<50F42F85.50907@xxxxxxxxxxxx>

Thanks, Bruce.
This looks like the old issue I was trying to fix with "SUNRPC: protect service sockets lists during per-net shutdown".
So, here is the problem as I see it: there is a transport, which is processed by service thread and it's processing is racing with per-net service shutdown:

CPU#0:							CPU#1:

svc_recv						svc_close_net
svc_get_next_xprt (list_del_init(xpt_ready))
							svc_close_list (set XPT_BUSY and XPT_CLOSE)
							svc_clear_pools(xprt was gained on CPU#0 already)
							svc_delete_xprt (set XPT_DEAD)
svc_handle_xprt (is XPT_CLOSE => svc_delete_xprt()
BUG()

So, from my POW, we need some way to:
1) Skip such in-progress transports on svc_close_net() call (there is not way to detect them, or at  least I don't see one)
2) Delete the transport after somewhere after svc_xprt_received()

But there is a problem with svc_xprt_received(): there is a call for svc_xprt_put() in it (svc_recv->svc_handle_xprt->svc_xprt_received->svc_xprt_put) . And if 
we are the only user - then the transport will be destroyed. But transport is dereferenced later in svc_recv() after the svc_handle_xprt call.

What do you think, Bruce?


> --b.


--
Best regards,
Stanislav Kinsbursky
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