linux-kernel - Re: [PATCH v2] SUNRPC: skip dead but not buried clients on PipeFS events

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Message-ID: <20120425173005.GD751@fieldses.org>
Date:	Wed, 25 Apr 2012 13:30:05 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	devel@...nvz.org
Subject: Re: [PATCH v2] SUNRPC: skip dead but not buried clients on PipeFS
 events

On Fri, Apr 20, 2012 at 06:11:02PM +0400, Stanislav Kinsbursky wrote:
> v2: atomic_inc_return() was replaced by atomic_inc_not_zero().
> 
> These clients can't be safely dereferenced if their counter in 0.

I'm pretty confused by how these notifiers work....

rpc_release_client decrements cl_count to zero temporarily, to have it
immediately re-incremented by rpc_free_auth.

So if we're called concurrently with rpc_release_client then it's sort
of random whether someone gets this callback.

Is that a problem?

Also, is this an existing bug?  (In which case Trond should take it
now.)

--b.

> 
> Signed-off-by: Stanislav Kinsbursky <skinsbursky@...allels.com>
> 
> ---
>  net/sunrpc/clnt.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> index 6797246..d10ebc4 100644
> --- a/net/sunrpc/clnt.c
> +++ b/net/sunrpc/clnt.c
> @@ -218,7 +218,8 @@ static struct rpc_clnt *rpc_get_client_for_event(struct net *net, int event)
>  		if (((event == RPC_PIPEFS_MOUNT) && clnt->cl_dentry) ||
>  		    ((event == RPC_PIPEFS_UMOUNT) && !clnt->cl_dentry))
>  			continue;
> -		atomic_inc(&clnt->cl_count);
> +		if (atomic_inc_not_zero(&clnt->cl_count) == 0)
> +			continue;
>  		spin_unlock(&sn->rpc_client_lock);
>  		return clnt;
>  	}
> 
--
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