NFSv4: fairly test all delegations on a SEQ4_ revocation

When the client is required to use TEST_STATEID to discover which
delegation(s) have been revoked, it may continually test delegations at the
head of the list if the server continues to be unsatisfied and send
SEQ4_STATUS_RECALLABLE_STATE_REVOKED. For a large number of delegations
this behavior is prone to live-lock because the client may never be able to
test and free revoked state at the end of the list since the
SEQ4_STATUS_RECALLABLE_STATE_REVOKED will cause us to flag delegations at
the head of the list to be tested. This problem is further exacerbated by
the state manager's willingness to be scheduled out on a busy system while
testing the list of delegations.

Keep a generation counter for each attempt to test all delegations, and
skip delegations that have already been tested in the current pass.

Signed-off-by: Benjamin Coddington <[e-mail]>
Tested-by: Torkil Svensgaard <[e-mail]>
Tested-by: Ruben Vestergaard <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/delegation.c

···
 	delegation->cred = get_cred(cred);
 	delegation->inode = inode;
 	delegation->flags = 1<<NFS_DELEGATION_REFERENCED;
+	delegation->test_gen = 0;
 	spin_lock_init(&delegation->lock);
 
 	spin_lock(&clp->cl_lock);
···
 	struct inode *inode;
 	const struct cred *cred;
 	nfs4_stateid stateid;
+	unsigned long gen = ++server->delegation_gen;
+
 restart:
 	rcu_read_lock();
 restart_locked:
···
 		    test_bit(NFS_DELEGATION_RETURNING,
 					&delegation->flags) ||
 		    test_bit(NFS_DELEGATION_TEST_EXPIRED,
-					&delegation->flags) == 0)
+					&delegation->flags) == 0 ||
+			delegation->test_gen == gen)
 			continue;
 		inode = nfs_delegation_grab_inode(delegation);
 		if (inode == NULL)
···
 		cred = get_cred_rcu(delegation->cred);
 		nfs4_stateid_copy(&stateid, &delegation->stateid);
 		spin_unlock(&delegation->lock);
+		delegation->test_gen = gen;
 		clear_bit(NFS_DELEGATION_TEST_EXPIRED, &delegation->flags);
 		rcu_read_unlock();
 		nfs_delegation_test_free_expired(inode, &stateid, cred);

fs/nfs/delegation.h

···
 	fmode_t type;
 	unsigned long pagemod_limit;
 	__u64 change_attr;
+	unsigned long test_gen;
 	unsigned long flags;
 	refcount_t refcount;
 	spinlock_t lock;

include/linux/nfs_fs_sb.h

···
 	struct list_head	delegations;
 	struct list_head	ss_copies;
 
+	unsigned long		delegation_gen;
 	unsigned long		mig_gen;
 	unsigned long		mig_status;
 #define NFS_MIG_IN_TRANSITION		(1)