netdev - Re: sunrpc: what prevents an xprt from being freed before task

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Date:	Tue, 03 Aug 2010 14:40:15 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-nfs@...r.kernel.org, netdev@...r.kernel.org,
	bfields@...ldses.org
Subject: Re: sunrpc: what prevents an xprt from being freed before
 task_cleanup runs?

On Tue, 2010-08-03 at 13:24 -0400, Jeff Layton wrote:
> We got a report recently about a panic in RHEL5 (2.6.18 based kernel).
> The problem appears to be that a task_cleanup workqueue job ran and got
> passed a pointer to an xprt that had been freed. The bug is here in
> case anyone is interested in the details:
> 
>     https://bugzilla.redhat.com/show_bug.cgi?id=611938
> 
> The situation seems to be pretty difficult to reproduce, but I don't
> see anything that's intended to ensure that this doesn't occur in RHEL5
> or mainline. The task_cleanup workqueue job doesn't hold a reference to
> the xprt, and the job isn't canceled when the xprt is torn down.
> 
> Bruce had a look and suggested that we may need something like the
> patch below (pasted in, so it probably won't apply correctly). I've
> tested a backported version of it on RHEL5 and it seems to work fine.
> 
> Is it reasonable to cancel task_cleanup when destroying the xprt? Or,
> am I missing something that should prevent this situation in mainline
> (and perhaps isn't in RHEL5's kernel).
> 
> Any help is appreciated...
> 
> -----------------------------[snip]---------------------------------
> diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> index dcd0132..2a1f664 100644
> --- a/net/sunrpc/xprt.c
> +++ b/net/sunrpc/xprt.c
> @@ -1129,6 +1129,7 @@ static void xprt_destroy(struct kref *kref)
>   rpc_destroy_wait_queue(&xprt->sending);
>   rpc_destroy_wait_queue(&xprt->resend);
>   rpc_destroy_wait_queue(&xprt->backlog);
> + cancel_work_sync(&xprt->task_cleanup);
>   /*
>    * Tear down transport state and free the rpc_xprt
>    */
> -----------------------------[snip]---------------------------------
> 
> Thanks,

How about doing a wait_on_bit_lock(&xprt->state, XPRT_LOCKED,...)
instead?

Cheers
   Trond

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