nfsd: convert nfs4_file searches to use RCU

The global state_lock protects the file_hashtbl, and that has the
potential to be a scalability bottleneck.

Address this by making the file_hashtbl use RCU. Add a rcu_head to the
nfs4_file and use that when freeing ones that have been hashed. In order
to conserve space, we union the fi_rcu field with the fi_delegations
list_head which must be clear by the time the last reference to the file
is dropped.

Convert find_file_locked to use RCU lookup primitives and not to require
that the state_lock be held, and convert find_file to do a lockless
lookup. Convert find_or_add_file to attempt a lockless lookup first, and
then fall back to doing a locked search and insert if that fails to find
anything.

Also, minimize the number of times we need to calculate the hash value
by passing it in as an argument to the search and insert functions, and
optimize the order of arguments in nfsd4_init_file.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -275,9 +275,11 @@ opaque_hashval(const void *ptr, int nbytes)
 	return x;
 }
 
-static void nfsd4_free_file(struct nfs4_file *f)
+static void nfsd4_free_file_rcu(struct rcu_head *rcu)
 {
-	kmem_cache_free(file_slab, f);
+	struct nfs4_file *fp = container_of(rcu, struct nfs4_file, fi_rcu);
+
+	kmem_cache_free(file_slab, fp);
 }
 
 static inline void
@@ -286,9 +288,10 @@ put_nfs4_file(struct nfs4_file *fi)
 	might_lock(&state_lock);
 
 	if (atomic_dec_and_lock(&fi->fi_ref, &state_lock)) {
-		hlist_del(&fi->fi_hash);
+		hlist_del_rcu(&fi->fi_hash);
 		spin_unlock(&state_lock);
-		nfsd4_free_file(fi);
+		WARN_ON_ONCE(!list_empty(&fi->fi_delegations));
+		call_rcu(&fi->fi_rcu, nfsd4_free_file_rcu);
 	}
 }
 
@@ -3057,10 +3060,9 @@ static struct nfs4_file *nfsd4_alloc_file(void)
 }
 
 /* OPEN Share state helper functions */
-static void nfsd4_init_file(struct nfs4_file *fp, struct knfsd_fh *fh)
+static void nfsd4_init_file(struct knfsd_fh *fh, unsigned int hashval,
+				struct nfs4_file *fp)
 {
-	unsigned int hashval = file_hashval(fh);
-
 	lockdep_assert_held(&state_lock);
 
 	atomic_set(&fp->fi_ref, 1);
@@ -3073,7 +3075,7 @@ static void nfsd4_init_file(struct nfs4_file *fp, struct knfsd_fh *fh)
 	fp->fi_share_deny = 0;
 	memset(fp->fi_fds, 0, sizeof(fp->fi_fds));
 	memset(fp->fi_access, 0, sizeof(fp->fi_access));
-	hlist_add_head(&fp->fi_hash, &file_hashtbl[hashval]);
+	hlist_add_head_rcu(&fp->fi_hash, &file_hashtbl[hashval]);
 }
 
 void
@@ -3294,17 +3296,14 @@ move_to_close_lru(struct nfs4_ol_stateid *s, struct net *net)
 
 /* search file_hashtbl[] for file */
 static struct nfs4_file *
-find_file_locked(struct knfsd_fh *fh)
+find_file_locked(struct knfsd_fh *fh, unsigned int hashval)
 {
-	unsigned int hashval = file_hashval(fh);
 	struct nfs4_file *fp;
 
-	lockdep_assert_held(&state_lock);
-
-	hlist_for_each_entry(fp, &file_hashtbl[hashval], fi_hash) {
+	hlist_for_each_entry_rcu(fp, &file_hashtbl[hashval], fi_hash) {
 		if (nfsd_fh_match(&fp->fi_fhandle, fh)) {
-			get_nfs4_file(fp);
-			return fp;
+			if (atomic_inc_not_zero(&fp->fi_ref))
+				return fp;
 		}
 	}
 	return NULL;
@@ -3314,22 +3313,30 @@ static struct nfs4_file *
 find_file(struct knfsd_fh *fh)
 {
 	struct nfs4_file *fp;
+	unsigned int hashval = file_hashval(fh);
 
-	spin_lock(&state_lock);
-	fp = find_file_locked(fh);
-	spin_unlock(&state_lock);
+	rcu_read_lock();
+	fp = find_file_locked(fh, hashval);
+	rcu_read_unlock();
 	return fp;
 }
 
 static struct nfs4_file *
 find_or_add_file(struct nfs4_file *new, struct knfsd_fh *fh)
 {
 	struct nfs4_file *fp;
+	unsigned int hashval = file_hashval(fh);
+
+	rcu_read_lock();
+	fp = find_file_locked(fh, hashval);
+	rcu_read_unlock();
+	if (fp)
+		return fp;
 
 	spin_lock(&state_lock);
-	fp = find_file_locked(fh);
-	if (fp == NULL) {
-		nfsd4_init_file(new, fh);
+	fp = find_file_locked(fh, hashval);
+	if (likely(fp == NULL)) {
+		nfsd4_init_file(fh, hashval, new);
 		fp = new;
 	}
 	spin_unlock(&state_lock);
@@ -4127,7 +4134,7 @@ void nfsd4_cleanup_open_state(struct nfsd4_compound_state *cstate,
 		nfs4_put_stateowner(so);
 	}
 	if (open->op_file)
-		nfsd4_free_file(open->op_file);
+		kmem_cache_free(file_slab, open->op_file);
 	if (open->op_stp)
 		nfs4_put_stid(&open->op_stp->st_stid);
 }

fs/nfsd/state.h

@@ -477,7 +477,10 @@ struct nfs4_file {
 	spinlock_t		fi_lock;
 	struct hlist_node       fi_hash;	/* hash on fi_fhandle */
 	struct list_head        fi_stateids;
-	struct list_head	fi_delegations;
+	union {
+		struct list_head	fi_delegations;
+		struct rcu_head		fi_rcu;
+	};
 	/* One each for O_RDONLY, O_WRONLY, O_RDWR: */
 	struct file *		fi_fds[3];
 	/*