nfsd: clients don't need to break their own delegations

We currently revoke read delegations on any write open or any operation
that modifies file data or metadata (including rename, link, and
unlink).  But if the delegation in question is the only read delegation
and is held by the client performing the operation, that's not really
necessary.

It's not always possible to prevent this in the NFSv4.0 case, because
there's not always a way to determine which client an NFSv4.0 delegation
came from.  (In theory we could try to guess this from the transport
layer, e.g., by assuming all traffic on a given TCP connection comes
from the same client.  But that's not really correct.)

In the NFSv4.1 case the session layer always tells us the client.

This patch should remove such self-conflicts in all cases where we can
reliably determine the client from the compound.

To do that we need to track "who" is performing a given (possibly
lease-breaking) file operation.  We're doing that by storing the
information in the svc_rqst and using kthread_data() to map the current
task back to a svc_rqst.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

Documentation/filesystems/locking.rst

@@ -425,6 +425,7 @@ prototypes::
 	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
 	void (*lm_break)(struct file_lock *); /* break_lease callback */
 	int (*lm_change)(struct file_lock **, int);
+	bool (*lm_breaker_owns_lease)(struct file_lock *);
 
 locking rules:
 
@@ -435,6 +436,7 @@ lm_notify:		yes		yes			no
 lm_grant:		no		no			no
 lm_break:		yes		no			no
 lm_change		yes		no			no
+lm_breaker_owns_lease:	no		no			no
 ==========		=============	=================	=========
 
 buffer_head

fs/locks.c

@@ -1557,6 +1557,9 @@ static bool leases_conflict(struct file_lock *lease, struct file_lock *breaker)
 {
 	bool rc;
 
+	if (lease->fl_lmops->lm_breaker_owns_lease
+			&& lease->fl_lmops->lm_breaker_owns_lease(lease))
+		return false;
 	if ((breaker->fl_flags & FL_LAYOUT) != (lease->fl_flags & FL_LAYOUT)) {
 		rc = false;
 		goto trace;

fs/nfsd/nfs4proc.c

@@ -2302,6 +2302,8 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 	}
 	check_if_stalefh_allowed(args);
 
+	rqstp->rq_lease_breaker = (void **)&cstate->clp;
+
 	trace_nfsd_compound(rqstp, args->opcnt);
 	while (!status && resp->opcnt < args->opcnt) {
 		op = &args->ops[resp->opcnt++];

fs/nfsd/nfs4state.c

@@ -4579,6 +4579,19 @@ nfsd_break_deleg_cb(struct file_lock *fl)
 	return ret;
 }
 
+static bool nfsd_breaker_owns_lease(struct file_lock *fl)
+{
+	struct nfs4_delegation *dl = fl->fl_owner;
+	struct svc_rqst *rqst;
+	struct nfs4_client *clp;
+
+	if (!i_am_nfsd())
+		return NULL;
+	rqst = kthread_data(current);
+	clp = *(rqst->rq_lease_breaker);
+	return dl->dl_stid.sc_client == clp;
+}
+
 static int
 nfsd_change_deleg_cb(struct file_lock *onlist, int arg,
 		     struct list_head *dispose)
@@ -4590,6 +4603,7 @@ nfsd_change_deleg_cb(struct file_lock *onlist, int arg,
 }
 
 static const struct lock_manager_operations nfsd_lease_mng_ops = {
+	.lm_breaker_owns_lease = nfsd_breaker_owns_lease,
 	.lm_break = nfsd_break_deleg_cb,
 	.lm_change = nfsd_change_deleg_cb,
 };

fs/nfsd/nfsd.h

@@ -88,6 +88,8 @@ int		nfsd_pool_stats_release(struct inode *, struct file *);
 
 void		nfsd_destroy(struct net *net);
 
+bool		i_am_nfsd(void);
+
 struct nfsdfs_client {
 	struct kref cl_ref;
 	void (*cl_release)(struct kref *kref);

fs/nfsd/nfssvc.c

@@ -601,6 +601,11 @@ static const struct svc_serv_ops nfsd_thread_sv_ops = {
 	.svo_module		= THIS_MODULE,
 };
 
+bool i_am_nfsd()
+{
+	return kthread_func(current) == nfsd;
+}
+
 int nfsd_create_serv(struct net *net)
 {
 	int error;
@@ -1011,6 +1016,7 @@ nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
 		*statp = rpc_garbage_args;
 		return 1;
 	}
+	rqstp->rq_lease_breaker = NULL;
 	/*
 	 * Give the xdr decoder a chance to change this if it wants
 	 * (necessary in the NFSv4.0 compound case)

include/linux/fs.h

@@ -1045,6 +1045,7 @@ struct lock_manager_operations {
 	bool (*lm_break)(struct file_lock *);
 	int (*lm_change)(struct file_lock *, int, struct list_head *);
 	void (*lm_setup)(struct file_lock *, void **);
+	bool (*lm_breaker_owns_lease)(struct file_lock *);
 };
 
 struct lock_manager {

include/linux/sunrpc/svc.h

@@ -299,6 +299,7 @@ struct svc_rqst {
 	struct net		*rq_bc_net;	/* pointer to backchannel's
 						 * net namespace
 						 */
+	void **			rq_lease_breaker; /* The v4 client breaking a lease */
 };
 
 #define SVC_NET(rqst) (rqst->rq_xprt ? rqst->rq_xprt->xpt_net : rqst->rq_bc_net)