NFSv4: Don't call put_rpccred() from an rcu callback

Doing so would require us to introduce bh-safe locks into put_rpccred().
This patch fixes the lockdep complaint reported by Marc Dietrich:

inconsistent {softirq-on-W} -> {in-softirq-W} usage.
swapper/0 [HC0[0]:SC1[1]:HE1:SE0] takes:
 (rpc_credcache_lock){-+..}, at: [<c01dc487>]
_atomic_dec_and_lock+0x17/0x60
{softirq-on-W} state was registered at:
  [<c013e870>] __lock_acquire+0x650/0x1030
  [<c013f2b1>] lock_acquire+0x61/0x80
  [<c02db9ac>] _spin_lock+0x2c/0x40
  [<c01dc487>] _atomic_dec_and_lock+0x17/0x60
  [<dced55fd>] put_rpccred+0x5d/0x100 [sunrpc]
  [<dced56c1>] rpcauth_unbindcred+0x21/0x60 [sunrpc]
  [<dced3fd4>] a0 [sunrpc]
  [<dcecefe0>] rpc_call_sync+0x30/0x40 [sunrpc]
  [<dcedc73b>] rpcb_register+0xdb/0x180 [sunrpc]
  [<dced65b3>] svc_register+0x93/0x160 [sunrpc]
  [<dced6ebe>] __svc_create+0x1ee/0x220 [sunrpc]
  [<dced7053>] svc_create+0x13/0x20 [sunrpc]
  [<dcf6d722>] nfs_callback_up+0x82/0x120 [nfs]
  [<dcf48f36>] nfs_get_client+0x176/0x390 [nfs]
  [<dcf49181>] nfs4_set_client+0x31/0x190 [nfs]
  [<dcf49983>] nfs4_create_server+0x63/0x3b0 [nfs]
  [<dcf52426>] nfs4_get_sb+0x346/0x5b0 [nfs]
  [<c017b444>] vfs_kern_mount+0x94/0x110
  [<c0190a62>] do_mount+0x1f2/0x7d0
  [<c01910a6>] sys_mount+0x66/0xa0
  [<c0104046>] syscall_call+0x7/0xb
  [<ffffffff>] 0xffffffff
irq event stamp: 5277830
hardirqs last  enabled at (5277830): [<c017530a>] kmem_cache_free+0x8a/0xc0
hardirqs last disabled at (5277829): [<c01752d2>] kmem_cache_free+0x52/0xc0
softirqs last  enabled at (5277798): [<c0124173>] __do_softirq+0xa3/0xc0
softirqs last disabled at (5277817): [<c01241d7>] do_softirq+0x47/0x50

other info that might help us debug this:
no locks held by swapper/0.

stack backtrace:
 [<c0104fda>] show_trace_log_lvl+0x1a/0x30
 [<c0105c02>] show_trace+0x12/0x20
 [<c0105d15>] dump_stack+0x15/0x20
 [<c013ccc3>] print_usage_bug+0x153/0x160
 [<c013d8b9>] mark_lock+0x449/0x620
 [<c013e824>] __lock_acquire+0x604/0x1030
 [<c013f2b1>] lock_acquire+0x61/0x80
 [<c02db9ac>] _spin_lock+0x2c/0x40
 [<c01dc487>] _atomic_dec_and_lock+0x17/0x60
 [<dced55fd>] put_rpccred+0x5d/0x100 [sunrpc]
 [<dcf6bf83>] nfs_free_delegation_callback+0x13/0x20 [nfs]
 [<c012f9ea>] __rcu_process_callbacks+0x6a/0x1c0
 [<c012fb52>] rcu_process_callbacks+0x12/0x30
 [<c0124218>] tasklet_action+0x38/0x80
 [<c0124125>] __do_softirq+0x55/0xc0
 [<c01241d7>] do_softirq+0x47/0x50
 [<c0124605>] irq_exit+0x35/0x40
 [<c0112463>] smp_apic_timer_interrupt+0x43/0x80
 [<c0104a77>] apic_timer_interrupt+0x33/0x38
 [<c02690df>] cpuidle_idle_call+0x6f/0x90
 [<c01023c3>] cpu_idle+0x43/0x70
 [<c02d8c27>] rest_init+0x47/0x50
 [<c03bcb6a>] start_kernel+0x22a/0x2b0
 [<00000000>] 0x0
 =======================
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/delegation.c

@@ -20,10 +20,8 @@
 #include "delegation.h"
 #include "internal.h"
 
-static void nfs_free_delegation(struct nfs_delegation *delegation)
+static void nfs_do_free_delegation(struct nfs_delegation *delegation)
 {
-	if (delegation->cred)
-		put_rpccred(delegation->cred);
 	kfree(delegation);
 }
 
@@ -31,7 +29,18 @@ static void nfs_free_delegation_callback(struct rcu_head *head)
 {
 	struct nfs_delegation *delegation = container_of(head, struct nfs_delegation, rcu);
 
-	nfs_free_delegation(delegation);
+	nfs_do_free_delegation(delegation);
+}
+
+static void nfs_free_delegation(struct nfs_delegation *delegation)
+{
+	struct rpc_cred *cred;
+
+	cred = rcu_dereference(delegation->cred);
+	rcu_assign_pointer(delegation->cred, NULL);
+	call_rcu(&delegation->rcu, nfs_free_delegation_callback);
+	if (cred)
+		put_rpccred(cred);
 }
 
 static int nfs_delegation_claim_locks(struct nfs_open_context *ctx, struct nfs4_state *state)
@@ -166,7 +175,7 @@ static int nfs_do_return_delegation(struct inode *inode, struct nfs_delegation *
 	int res = 0;
 
 	res = nfs4_proc_delegreturn(inode, delegation->cred, &delegation->stateid);
-	call_rcu(&delegation->rcu, nfs_free_delegation_callback);
+	nfs_free_delegation(delegation);
 	return res;
 }
 
@@ -448,7 +457,7 @@ restart:
 		spin_unlock(&clp->cl_lock);
 		rcu_read_unlock();
 		if (delegation != NULL)
-			call_rcu(&delegation->rcu, nfs_free_delegation_callback);
+			nfs_free_delegation(delegation);
 		goto restart;
 	}
 	rcu_read_unlock();