[PATCH] NFSv4: Fix up RCU annotations for struct nfs_netns_client

[trondmy@kernel.org]

From: Trond Myklebust <trond.myklebust@hammerspace.com>

The identifier is read as an RCU protected string. Its value may
be changed during the lifetime of the network namespace by writing
a new string into the sysfs pseudofile (at which point, we free the
old string only after a call to synchronize_rcu()).

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 fs/nfs/sysfs.c | 11 ++++++++---
 fs/nfs/sysfs.h |  2 +-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/nfs/sysfs.c b/fs/nfs/sysfs.c
index c489496b5659..8cb70755e3c9 100644
--- a/fs/nfs/sysfs.c
+++ b/fs/nfs/sysfs.c
@@ -79,7 +79,12 @@ static ssize_t nfs_netns_identifier_show(struct kobject *kobj,
 	struct nfs_netns_client *c = container_of(kobj,
 			struct nfs_netns_client,
 			kobject);
-	return scnprintf(buf, PAGE_SIZE, "%s\n", c->identifier);
+	ssize_t ret;
+
+	rcu_read_lock();
+	ret = scnprintf(buf, PAGE_SIZE, "%s\n", rcu_dereference(c->identifier));
+	rcu_read_unlock();
+	return ret;
 }
 
 /* Strip trailing '\n' */
@@ -107,7 +112,7 @@ static ssize_t nfs_netns_identifier_store(struct kobject *kobj,
 	p = kmemdup_nul(buf, len, GFP_KERNEL);
 	if (!p)
 		return -ENOMEM;
-	old = xchg(&c->identifier, p);
+	old = rcu_dereference_protected(xchg(&c->identifier, (char __rcu *)p), 1);
 	if (old) {
 		synchronize_rcu();
 		kfree(old);
@@ -121,7 +126,7 @@ static void nfs_netns_client_release(struct kobject *kobj)
 			struct nfs_netns_client,
 			kobject);
 
-	kfree(c->identifier);
+	kfree(rcu_dereference_raw(c->identifier));
 	kfree(c);
 }
 
diff --git a/fs/nfs/sysfs.h b/fs/nfs/sysfs.h
index ebcbdc40483b..5501ef573c32 100644
--- a/fs/nfs/sysfs.h
+++ b/fs/nfs/sysfs.h
@@ -11,7 +11,7 @@
 struct nfs_netns_client {
 	struct kobject kobject;
 	struct net *net;
-	const char *identifier;
+	const char __rcu *identifier;
 };
 
 extern struct kobject *nfs_client_kobj;
-- 
2.26.2