nfsd: don't call nfsd_file_put from client states seqfile display

We had a report of this:

    BUG: sleeping function called from invalid context at fs/nfsd/filecache.c:440

...with a stack trace showing nfsd_file_put being called from
nfs4_show_open. This code has always tried to call fput while holding a
spinlock, but we recently changed this to use the filecache, and that
started triggering the might_sleep() in nfsd_file_put.

states_start takes and holds the cl_lock while iterating over the
client's states, and we can't sleep with that held.

Have the various nfs4_show_* functions instead hold the fi_lock instead
of taking a nfsd_file reference.

Fixes: 78599c42 ("nfsd4: add file to display list of client's opens")
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2138357Reported-by: Zhi Li <yieli@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -675,15 +675,26 @@ find_any_file(struct nfs4_file *f)
 	return ret;
 }
 
-static struct nfsd_file *find_deleg_file(struct nfs4_file *f)
+static struct nfsd_file *find_any_file_locked(struct nfs4_file *f)
 {
-	struct nfsd_file *ret = NULL;
+	lockdep_assert_held(&f->fi_lock);
+
+	if (f->fi_fds[O_RDWR])
+		return f->fi_fds[O_RDWR];
+	if (f->fi_fds[O_WRONLY])
+		return f->fi_fds[O_WRONLY];
+	if (f->fi_fds[O_RDONLY])
+		return f->fi_fds[O_RDONLY];
+	return NULL;
+}
+
+static struct nfsd_file *find_deleg_file_locked(struct nfs4_file *f)
+{
+	lockdep_assert_held(&f->fi_lock);
 
-	spin_lock(&f->fi_lock);
 	if (f->fi_deleg_file)
-		ret = nfsd_file_get(f->fi_deleg_file);
-	spin_unlock(&f->fi_lock);
-	return ret;
+		return f->fi_deleg_file;
+	return NULL;
 }
 
 static atomic_long_t num_delegations;
@@ -2613,9 +2624,11 @@ static int nfs4_show_open(struct seq_file *s, struct nfs4_stid *st)
 	ols = openlockstateid(st);
 	oo = ols->st_stateowner;
 	nf = st->sc_file;
-	file = find_any_file(nf);
+
+	spin_lock(&nf->fi_lock);
+	file = find_any_file_locked(nf);
 	if (!file)
-		return 0;
+		goto out;
 
 	seq_printf(s, "- ");
 	nfs4_show_stateid(s, &st->sc_stateid);
@@ -2637,8 +2650,8 @@ static int nfs4_show_open(struct seq_file *s, struct nfs4_stid *st)
 	seq_printf(s, ", ");
 	nfs4_show_owner(s, oo);
 	seq_printf(s, " }\n");
-	nfsd_file_put(file);
-
+out:
+	spin_unlock(&nf->fi_lock);
 	return 0;
 }
 
@@ -2652,9 +2665,10 @@ static int nfs4_show_lock(struct seq_file *s, struct nfs4_stid *st)
 	ols = openlockstateid(st);
 	oo = ols->st_stateowner;
 	nf = st->sc_file;
-	file = find_any_file(nf);
+	spin_lock(&nf->fi_lock);
+	file = find_any_file_locked(nf);
 	if (!file)
-		return 0;
+		goto out;
 
 	seq_printf(s, "- ");
 	nfs4_show_stateid(s, &st->sc_stateid);
@@ -2674,8 +2688,8 @@ static int nfs4_show_lock(struct seq_file *s, struct nfs4_stid *st)
 	seq_printf(s, ", ");
 	nfs4_show_owner(s, oo);
 	seq_printf(s, " }\n");
-	nfsd_file_put(file);
-
+out:
+	spin_unlock(&nf->fi_lock);
 	return 0;
 }
 
@@ -2687,9 +2701,10 @@ static int nfs4_show_deleg(struct seq_file *s, struct nfs4_stid *st)
 
 	ds = delegstateid(st);
 	nf = st->sc_file;
-	file = find_deleg_file(nf);
+	spin_lock(&nf->fi_lock);
+	file = find_deleg_file_locked(nf);
 	if (!file)
-		return 0;
+		goto out;
 
 	seq_printf(s, "- ");
 	nfs4_show_stateid(s, &st->sc_stateid);
@@ -2705,8 +2720,8 @@ static int nfs4_show_deleg(struct seq_file *s, struct nfs4_stid *st)
 	seq_printf(s, ", ");
 	nfs4_show_fname(s, file);
 	seq_printf(s, " }\n");
-	nfsd_file_put(file);
-
+out:
+	spin_unlock(&nf->fi_lock);
 	return 0;
 }