linux-kernel - Re: strange put

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Date:	Sun, 27 Mar 2011 17:33:46 +0200
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: strange put_rpccred() handling

On Mon, 2011-03-28 at 00:05 +0900, OGAWA Hirofumi wrote:
> Hi,
> 
> BUG: atomic_dec_and_test(): -1: atomic counter underflow at:
> Pid: 2827, comm: mount.nfs Not tainted 2.6.38 #1
> Call Trace:
>  [<ffffffffa02223a0>] ? put_rpccred+0x44/0x14e [sunrpc]
>  [<ffffffffa021bbe9>] ? rpc_ping+0x4e/0x58 [sunrpc]
>  [<ffffffffa021c4a5>] ? rpc_create+0x481/0x4fc [sunrpc]
>  [<ffffffffa022298a>] ? rpcauth_lookup_credcache+0xab/0x22d [sunrpc]
>  [<ffffffffa028be8c>] ? nfs_create_rpc_client+0xa6/0xeb [nfs]
>  [<ffffffffa028c660>] ? nfs4_set_client+0xc2/0x1f9 [nfs]
>  [<ffffffffa028cd3c>] ? nfs4_create_server+0xf2/0x2a6 [nfs]
>  [<ffffffffa0295d07>] ? nfs4_remote_mount+0x4e/0x14a [nfs]
>  [<ffffffff810dd570>] ? vfs_kern_mount+0x6e/0x133
>  [<ffffffffa029605a>] ? nfs_do_root_mount+0x76/0x95 [nfs]
>  [<ffffffffa029643d>] ? nfs4_try_mount+0x56/0xaf [nfs]
>  [<ffffffffa0297434>] ? nfs_get_sb+0x435/0x73c [nfs]
>  [<ffffffff810dd59b>] ? vfs_kern_mount+0x99/0x133
>  [<ffffffff810dd693>] ? do_kern_mount+0x48/0xd8
>  [<ffffffff810f5b75>] ? do_mount+0x6da/0x741
>  [<ffffffff810f5c5f>] ? sys_mount+0x83/0xc0
>  [<ffffffff8100293b>] ? system_call_fastpath+0x16/0x1b
> 
> This is not oops, and debug code is not in vanilla. This debug code is
> simple - detects atomic_dec_and_test() underflow.
> 
> Well, so, I think this is real bug of nfs codes somewhere. With some
> review, the code
> 
> rpc_call_sync()
>     rpc_run_task
>         rpc_execute()
>             __rpc_execute()
>                 rpc_release_task()
>                     rpc_release_resources_task()
>                         put_rpccred()                <= release cred
>     rpc_put_task
>         rpc_do_put_task()
>             rpc_release_resources_task()
>                 put_rpccred()                        <= release cred again
> 
> seems to be release cred unintendedly.
> 
> static void rpc_release_resources_task(struct rpc_task *task)
> {
> 	if (task->tk_rqstp)
> 		xprt_release(task);
> 	if (task->tk_msg.rpc_cred) {
> 		put_rpccred(task->tk_msg.rpc_cred);
> 		task->tk_msg.rpc_cred = NULL;
> 	}
> 	rpc_task_release_client(task);
> }
> 
> The above change may fix the problem though, I don't know the codes what
> want to do actually. And I guess this is not right fix, because the path
> is looks strange - on early stage, __rpc_execute() calls
> rpc_release_task() explicitly.

Argh! You are completely correct. The intention was that
rpc_release_resources_task() should be able to be called more than once,
which means that we have to set task->tk_msg.rpc_cred to NULL after
freeing the cred. I've no idea why I missed that when I cleaned up that
code.

Thanks for debugging this!

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

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