linux-kernel - Question about nfs4_destroy

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Message-ID: <20140212214209.GA4136@linux.vnet.ibm.com>
Date:	Wed, 12 Feb 2014 13:42:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	trond.myklebust@...marydata.com
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Question about nfs4_destroy_session()

Hello, Trond,

In nfs4_destroy_session(), there is an rcu_dereference() that looks to
leak the returned pointer out of an RCU read-side critical section.
If the pointed-to object might have just now been created, this is a
bug because xprt_destroy_backchannel() dereferences this pointer.

So, does xprt_destroy_backchannel() exclude creation-side code?  (If so,
no bug -- but a comment might be good.)

							Thanx, Paul

void nfs4_destroy_session(struct nfs4_session *session)
{
	struct rpc_xprt *xprt;
	struct rpc_cred *cred;

	cred = nfs4_get_clid_cred(session->clp);
	nfs4_proc_destroy_session(session, cred);
	if (cred)
		put_rpccred(cred);

	rcu_read_lock();
	xprt = rcu_dereference(session->clp->cl_rpcclient->cl_xprt);
	rcu_read_unlock();
	dprintk("%s Destroy backchannel for xprt %p\n",
		__func__, xprt);
	xprt_destroy_backchannel(xprt, NFS41_BC_MIN_CALLBACKS);
	nfs4_destroy_session_slot_tables(session);
	kfree(session);
}

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