linux-kernel - Re: BUG at net/sunrpc/svc

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Date:	Thu, 17 Jan 2013 18:41:08 -0500
From:	Mark Lord <kernel@...savvy.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-nfs@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: BUG at net/sunrpc/svc_xprt.c:921

On 13-01-17 08:24 AM, Stanislav Kinsbursky wrote:
..
> 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.

Sounds like a reference count type of problem/solution (kref) (?)

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