nfsd: add nfsd_serv_try_get and nfsd_serv_put

Introduce nfsd_serv_try_get and nfsd_serv_put and update the nfsd code
to prevent nfsd_destroy_serv from destroying nn->nfsd_serv until any
caller of nfsd_serv_try_get releases their reference using nfsd_serv_put.

A percpu_ref is used to implement the interlock between
nfsd_destroy_serv and any caller of nfsd_serv_try_get.

This interlock is needed to properly wait for the completion of client
initiated localio calls to nfsd (that are _not_ in the context of nfsd).
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

fs/nfsd/netns.h

@@ -13,6 +13,7 @@
 #include <linux/filelock.h>
 #include <linux/nfs4.h>
 #include <linux/percpu_counter.h>
+#include <linux/percpu-refcount.h>
 #include <linux/siphash.h>
 #include <linux/sunrpc/stats.h>
 
@@ -139,7 +140,9 @@ struct nfsd_net {
 
 	struct svc_info nfsd_info;
 #define nfsd_serv nfsd_info.serv
-
+	struct percpu_ref nfsd_serv_ref;
+	struct completion nfsd_serv_confirm_done;
+	struct completion nfsd_serv_free_done;
 
 	/*
 	 * clientid and stateid data for construction of net unique COPY
@@ -222,6 +225,9 @@ struct nfsd_net {
 extern bool nfsd_support_version(int vers);
 extern unsigned int nfsd_net_id;
 
+bool nfsd_serv_try_get(struct net *net);
+void nfsd_serv_put(struct net *net);
+
 void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn);
 void nfsd_reset_write_verifier(struct nfsd_net *nn);
 #endif /* __NFSD_NETNS_H__ */

fs/nfsd/nfssvc.c

@@ -193,6 +193,34 @@ int nfsd_minorversion(struct nfsd_net *nn, u32 minorversion, enum vers_op change
 	return 0;
 }
 
+bool nfsd_serv_try_get(struct net *net)
+{
+	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+
+	return (nn && percpu_ref_tryget_live(&nn->nfsd_serv_ref));
+}
+
+void nfsd_serv_put(struct net *net)
+{
+	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+
+	percpu_ref_put(&nn->nfsd_serv_ref);
+}
+
+static void nfsd_serv_done(struct percpu_ref *ref)
+{
+	struct nfsd_net *nn = container_of(ref, struct nfsd_net, nfsd_serv_ref);
+
+	complete(&nn->nfsd_serv_confirm_done);
+}
+
+static void nfsd_serv_free(struct percpu_ref *ref)
+{
+	struct nfsd_net *nn = container_of(ref, struct nfsd_net, nfsd_serv_ref);
+
+	complete(&nn->nfsd_serv_free_done);
+}
+
 /*
  * Maximum number of nfsd processes
  */
@@ -392,6 +420,7 @@ static void nfsd_shutdown_net(struct net *net)
 		lockd_down(net);
 		nn->lockd_up = false;
 	}
+	percpu_ref_exit(&nn->nfsd_serv_ref);
 	nn->nfsd_net_up = false;
 	nfsd_shutdown_generic();
 }
@@ -471,6 +500,13 @@ void nfsd_destroy_serv(struct net *net)
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 	struct svc_serv *serv = nn->nfsd_serv;
 
+	lockdep_assert_held(&nfsd_mutex);
+
+	percpu_ref_kill_and_confirm(&nn->nfsd_serv_ref, nfsd_serv_done);
+	wait_for_completion(&nn->nfsd_serv_confirm_done);
+	wait_for_completion(&nn->nfsd_serv_free_done);
+	/* percpu_ref_exit is called in nfsd_shutdown_net */
+
 	spin_lock(&nfsd_notifier_lock);
 	nn->nfsd_serv = NULL;
 	spin_unlock(&nfsd_notifier_lock);
@@ -595,6 +631,13 @@ int nfsd_create_serv(struct net *net)
 	if (nn->nfsd_serv)
 		return 0;
 
+	error = percpu_ref_init(&nn->nfsd_serv_ref, nfsd_serv_free,
+				0, GFP_KERNEL);
+	if (error)
+		return error;
+	init_completion(&nn->nfsd_serv_free_done);
+	init_completion(&nn->nfsd_serv_confirm_done);
+
 	if (nfsd_max_blksize == 0)
 		nfsd_max_blksize = nfsd_get_default_max_blksize();
 	nfsd_reset_versions(nn);