[PATCH v5 3/4] kernfs: Introduce interface to access global kernfs_open_file_mutex.

[Imran Khan <imran.f.khan@oracle.com>]

This allows to change underlying mutex locking, without needing to change
the users of the lock. For example next patch modifies this interface to
use hashed mutexes in place of a single global kernfs_open_file_mutex.

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Acked-by: Tejun Heo <tj@kernel.org>
---
 fs/kernfs/file.c | 60 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 40 insertions(+), 20 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 0cc288dcf51d9..205b5c71f30cb 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -49,6 +49,22 @@ struct kernfs_open_node {
 
 static LLIST_HEAD(kernfs_notify_list);
 
+static inline struct mutex *kernfs_open_file_mutex_ptr(struct kernfs_node *kn)
+{
+	return &kernfs_open_file_mutex;
+}
+
+static inline struct mutex *kernfs_open_file_mutex_lock(struct kernfs_node *kn)
+{
+	struct mutex *lock;
+
+	lock = kernfs_open_file_mutex_ptr(kn);
+
+	mutex_lock(lock);
+
+	return lock;
+}
+
 /**
  * kernfs_deref_open_node - Get kernfs_open_node corresponding to @kn.
  *
@@ -56,7 +72,7 @@ static LLIST_HEAD(kernfs_notify_list);
  * @kn: target kernfs_node.
  *
  * Fetch and return ->attr.open of @kn if @of->list is not empty or if
- * kernfs_open_file_mutex is held.
+ * kernfs_open_file_mutex_ptr(kn) is held.
  *
  * We can rely on either of the following 2 conditions to ensure that
  * dereferencing ->attr.open outside RCU read-side critical section is safe.
@@ -68,13 +84,13 @@ static LLIST_HEAD(kernfs_notify_list);
  *
  * or
  *
- * 2. Update of ->attr.open happens under kernfs_open_file_mutex. So as long as
- * the current updater (caller) is holding this mutex, other updaters will not
- * be able to change ->attr.open and this means that we can safely deref
- * ->attr.open outside RCU read-side critical section.
+ * 2. Update of ->attr.open happens under kernfs_open_file_mutex_ptr(kn). So as
+ * long as the current updater (caller) holds this mutex, other updaters can't
+ * change ->attr.open and this means that we can safely deref ->attr.open
+ * outside RCU read-side critical section.
  *
  * The caller needs to make sure that either @of->list is not empty or
- * kernfs_open_file_mutex is held.
+ * kernfs_open_file_mutex_ptr(kn) is held.
  */
 static struct kernfs_open_node *
 kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
@@ -83,7 +99,7 @@ kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
 	bool deref_ok;
 
 	deref_ok = (of ? !list_empty(&of->list) : false) ||
-		   lockdep_is_held(&kernfs_open_file_mutex);
+		   lockdep_is_held(kernfs_open_file_mutex_ptr(kn));
 
 	on = rcu_dereference_check(kn->attr.open, deref_ok);
 
@@ -570,19 +586,20 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
 				struct kernfs_open_file *of)
 {
 	struct kernfs_open_node *on, *new_on = NULL;
+	struct mutex *mutex = NULL;
 
-	mutex_lock(&kernfs_open_file_mutex);
+	mutex = kernfs_open_file_mutex_lock(kn);
 	on = kernfs_deref_open_node(NULL, kn);
 
 	if (on) {
 		list_add_tail(&of->list, &on->files);
-		mutex_unlock(&kernfs_open_file_mutex);
+		mutex_unlock(mutex);
 		return 0;
 	} else {
 		/* not there, initialize a new one */
 		new_on = kmalloc(sizeof(*new_on), GFP_KERNEL);
 		if (!new_on) {
-			mutex_unlock(&kernfs_open_file_mutex);
+			mutex_unlock(mutex);
 			return -ENOMEM;
 		}
 		atomic_set(&new_on->event, 1);
@@ -591,7 +608,7 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
 		list_add_tail(&of->list, &new_on->files);
 		rcu_assign_pointer(kn->attr.open, new_on);
 	}
-	mutex_unlock(&kernfs_open_file_mutex);
+	mutex_unlock(mutex);
 
 	return 0;
 }
@@ -613,12 +630,13 @@ static void kernfs_unlink_open_file(struct kernfs_node *kn,
 				 struct kernfs_open_file *of)
 {
 	struct kernfs_open_node *on;
+	struct mutex *mutex = NULL;
 
-	mutex_lock(&kernfs_open_file_mutex);
+	mutex = kernfs_open_file_mutex_lock(kn);
 
 	on = kernfs_deref_open_node(NULL, kn);
 	if (!on) {
-		mutex_unlock(&kernfs_open_file_mutex);
+		mutex_unlock(mutex);
 		return;
 	}
 
@@ -630,7 +648,7 @@ static void kernfs_unlink_open_file(struct kernfs_node *kn,
 		kfree_rcu(on, rcu_head);
 	}
 
-	mutex_unlock(&kernfs_open_file_mutex);
+	mutex_unlock(mutex);
 }
 
 static int kernfs_fop_open(struct inode *inode, struct file *file)
@@ -772,7 +790,7 @@ static void kernfs_release_file(struct kernfs_node *kn,
 	 * here because drain path may be called from places which can
 	 * cause circular dependency.
 	 */
-	lockdep_assert_held(&kernfs_open_file_mutex);
+	lockdep_assert_held(kernfs_open_file_mutex_ptr(kn));
 
 	if (!of->released) {
 		/*
@@ -789,11 +807,12 @@ static int kernfs_fop_release(struct inode *inode, struct file *filp)
 {
 	struct kernfs_node *kn = inode->i_private;
 	struct kernfs_open_file *of = kernfs_of(filp);
+	struct mutex *mutex = NULL;
 
 	if (kn->flags & KERNFS_HAS_RELEASE) {
-		mutex_lock(&kernfs_open_file_mutex);
+		mutex = kernfs_open_file_mutex_lock(kn);
 		kernfs_release_file(kn, of);
-		mutex_unlock(&kernfs_open_file_mutex);
+		mutex_unlock(mutex);
 	}
 
 	kernfs_unlink_open_file(kn, of);
@@ -808,6 +827,7 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
 {
 	struct kernfs_open_node *on;
 	struct kernfs_open_file *of;
+	struct mutex *mutex = NULL;
 
 	if (!(kn->flags & (KERNFS_HAS_MMAP | KERNFS_HAS_RELEASE)))
 		return;
@@ -823,10 +843,10 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
 	if (!rcu_access_pointer(kn->attr.open))
 		return;
 
-	mutex_lock(&kernfs_open_file_mutex);
+	mutex = kernfs_open_file_mutex_lock(kn);
 	on = kernfs_deref_open_node(NULL, kn);
 	if (!on) {
-		mutex_unlock(&kernfs_open_file_mutex);
+		mutex_unlock(mutex);
 		return;
 	}
 
@@ -840,7 +860,7 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
 			kernfs_release_file(kn, of);
 	}
 
-	mutex_unlock(&kernfs_open_file_mutex);
+	mutex_unlock(mutex);
 }
 
 /*
-- 
2.30.2