linux-kernel - strange put_rpccred() handling

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Date:	Mon, 28 Mar 2011 00:05:02 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	"J. Bruce Fields" <bfields@...ldses.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: strange put_rpccred() handling

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.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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