[RFC PATCH 10/14] nfsd: keep a reference to the fs_struct in svc_rqst

[Jeff Layton <jlayton@primarydata.com>]

When we convert this code to use a workqueue, we won't want to allocate
a new fs_struct to handle each RPC. Doing so might also be problematic
since we'd be swapping out the ->fs value on a "public" workqueue
kthread.

Change the code to allocate an fs struct when when allocating a svc_rqst
and then switch to using that in the "nfsd" function.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
---
 fs/fs_struct.c             | 59 +++++++++++++++++++++++++++++++++++++++-------
 fs/nfsd/nfssvc.c           | 17 ++++++-------
 include/linux/fs_struct.h  |  3 +++
 include/linux/sunrpc/svc.h |  1 +
 net/sunrpc/svc.c           |  8 +++++++
 5 files changed, 69 insertions(+), 19 deletions(-)

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 7dca743b2ce1..9bc08ea2f433 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -127,26 +127,67 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
 	}
 	return fs;
 }
+EXPORT_SYMBOL_GPL(copy_fs_struct);
 
-int unshare_fs_struct(void)
+/* Replace current fs struct with one given. Return a pointer to old one. */
+static struct fs_struct *
+swap_fs_struct(struct fs_struct *new_fs)
 {
-	struct fs_struct *fs = current->fs;
-	struct fs_struct *new_fs = copy_fs_struct(fs);
-	int kill;
-
-	if (!new_fs)
-		return -ENOMEM;
+	struct fs_struct *old_fs;
 
 	task_lock(current);
+	old_fs = current->fs;
+	current->fs = new_fs;
+	task_unlock(current);
+
+	return old_fs;
+}
+
+/* Put a reference to a fs_struct. */
+void put_fs_struct(struct fs_struct *fs)
+{
+	bool kill;
+
 	spin_lock(&fs->lock);
 	kill = !--fs->users;
-	current->fs = new_fs;
 	spin_unlock(&fs->lock);
-	task_unlock(current);
 
 	if (kill)
 		free_fs_struct(fs);
+}
+EXPORT_SYMBOL_GPL(put_fs_struct);
+
+/* Take an extra reference to a fs_struct. Caller must already hold one! */
+struct fs_struct *
+get_fs_struct(struct fs_struct *fs)
+{
+	spin_lock(&fs->lock);
+	++fs->users;
+	spin_unlock(&fs->lock);
+	return fs;
+}
+EXPORT_SYMBOL_GPL(get_fs_struct);
+
+/*
+ * Swap in a new fs_struct and drop the reference on the old one.
+ * Caller must have already taken the reference to the new one.
+ */
+void replace_fs_struct(struct fs_struct *new_fs)
+{
+	struct fs_struct *old_fs = swap_fs_struct(new_fs);
+
+	put_fs_struct(old_fs);
+}
+EXPORT_SYMBOL_GPL(replace_fs_struct);
+
+int unshare_fs_struct(void)
+{
+	struct fs_struct *new_fs = copy_fs_struct(current->fs);
+
+	if (!new_fs)
+		return -ENOMEM;
 
+	replace_fs_struct(new_fs);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(unshare_fs_struct);
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 71e7b180c0d9..f37bd7db2176 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -582,15 +582,13 @@ nfsd(void *vrqstp)
 	/* Lock module and set up kernel thread */
 	mutex_lock(&nfsd_mutex);
 
-	/* At this point, the thread shares current->fs
-	 * with the init process. We need to create files with a
-	 * umask of 0 instead of init's umask. */
-	if (unshare_fs_struct() < 0) {
-		printk("Unable to start nfsd thread: out of memory\n");
-		goto out;
-	}
-
-	current->fs->umask = 0;
+	/*
+	 * At this point, the thread shares current->fs with the init process.
+	 * We need to create files with a umask of 0 instead of init's umask,
+	 * so switch to the fs_struct associated with the rqstp.
+	 */
+	get_fs_struct(rqstp->rq_fs);
+	replace_fs_struct(rqstp->rq_fs);
 
 	/*
 	 * thread is spawned with all signals set to SIG_IGN, re-enable
@@ -632,7 +630,6 @@ nfsd(void *vrqstp)
 	mutex_lock(&nfsd_mutex);
 	nfsdstats.th_cnt --;
 
-out:
 	rqstp->rq_server = NULL;
 
 	/* Release the thread */
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index 0efc3e62843a..d2b7a1942790 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -21,7 +21,10 @@ extern void set_fs_root(struct fs_struct *, const struct path *);
 extern void set_fs_pwd(struct fs_struct *, const struct path *);
 extern struct fs_struct *copy_fs_struct(struct fs_struct *);
 extern void free_fs_struct(struct fs_struct *);
+extern void replace_fs_struct(struct fs_struct *);
 extern int unshare_fs_struct(void);
+struct fs_struct *get_fs_struct(struct fs_struct *);
+void put_fs_struct(struct fs_struct *);
 
 static inline void get_fs_root(struct fs_struct *fs, struct path *root)
 {
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 81e723220346..f47de87660b4 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -295,6 +295,7 @@ struct svc_rqst {
 	struct svc_cacherep *	rq_cacherep;	/* cache info */
 	struct task_struct	*rq_task;	/* service thread */
 	spinlock_t		rq_lock;	/* per-request lock */
+	struct fs_struct 	*rq_fs;
 };
 
 #define SVC_NET(svc_rqst)	(svc_rqst->rq_xprt->xpt_net)
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 7c8e33923210..4300bc852f6e 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -20,6 +20,7 @@
 #include <linux/module.h>
 #include <linux/kthread.h>
 #include <linux/slab.h>
+#include <linux/fs_struct.h>
 
 #include <linux/sunrpc/types.h>
 #include <linux/sunrpc/xdr.h>
@@ -622,6 +623,11 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
 	if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
 		goto out_enomem;
 
+	rqstp->rq_fs = copy_fs_struct(current->fs);
+	if (!rqstp->rq_fs)
+		goto out_enomem;
+
+	rqstp->rq_fs->umask = 0;
 	return rqstp;
 out_enomem:
 	svc_rqst_free(rqstp);
@@ -784,6 +790,8 @@ svc_rqst_free(struct svc_rqst *rqstp)
 	kfree(rqstp->rq_resp);
 	kfree(rqstp->rq_argp);
 	kfree(rqstp->rq_auth_data);
+	if (rqstp->rq_fs)
+		put_fs_struct(rqstp->rq_fs);
 	kfree_rcu(rqstp, rq_rcu_head);
 }
 EXPORT_SYMBOL_GPL(svc_rqst_free);
-- 
2.1.0