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From:   Imran Khan <imran.f.khan@oracle.com>
To:     tj@kernel.org, viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org
Cc:     matthew.wilcox@oracle.com, konrad.wilk@oracle.com,
        linux-kernel@vger.kernel.org
Subject: [PATCH RESEND v4 3/4] kernfs: Introduce interface to access global kernfs_open_file_mutex.
Date:   Thu,  2 Jun 2022 16:39:06 +1000
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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 | 72 +++++++++++++++++++++++++++++++-----------------
 1 file changed, 46 insertions(+), 26 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index b21c8e3b6a8d..d35d01d30fa0 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.
  *
@@ -80,21 +96,21 @@ kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
  * @kn: target kernfs_node.
  *
  * Fetch and return ->attr.open of @kn when caller(writer) holds
- * kernfs_open_file_mutex.
+ * kernfs_open_file_mutex_ptr(kn).
  *
- * 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.
+ * Update of ->attr.open happens under kernfs_open_file_mutex_ptr(kn). 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.
  *
  * This should ONLY be used by updaters of ->attr.open and caller needs to make
- * sure that kernfs_open_file_mutex is held.
+ * sure that kernfs_open_file_mutex_ptr(kn) is held.
  */
 static struct kernfs_open_node *
 kernfs_deref_open_node_protected(struct kernfs_node *kn)
 {
 	return rcu_dereference_protected(kn->attr.open,
-				lockdep_is_held(&kernfs_open_file_mutex));
+			lockdep_is_held(kernfs_open_file_mutex_ptr(kn)));
 }
 
 /**
@@ -103,21 +119,21 @@ kernfs_deref_open_node_protected(struct kernfs_node *kn)
  * @kn: target kernfs_node.
  *
  * Fetch and return ->attr.open of @kn when caller(reader) holds
- * kernfs_open_file_mutex.
+ * kernfs_open_file_mutex_ptr(kn).
  *
- * Update of ->attr.open happens under kernfs_open_file_mutex. So as long as
- * the current reader (caller) is holding this mutex, updaters will not be
- * able to change ->attr.open and this means that we can safely deref
+ * Update of ->attr.open happens under kernfs_open_file_mutex_ptr(kn). So as
+ * long as the current reader (caller) is holding this mutex, 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.
  *
  * This should ONLY be used by readers of ->attr.open and caller needs to make
- * sure that kernfs_open_file_mutex is held.
+ * sure that kernfs_open_file_mutex_ptr(kn) is held.
  */
 static struct kernfs_open_node *
 kernfs_check_open_node_protected(struct kernfs_node *kn)
 {
 	return rcu_dereference_check(kn->attr.open,
-				      lockdep_is_held(&kernfs_open_file_mutex));
+			lockdep_is_held(kernfs_open_file_mutex_ptr(kn)));
 }
 
 static struct kernfs_open_file *kernfs_of(struct file *file)
@@ -600,19 +616,20 @@ 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_protected(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);
@@ -621,7 +638,7 @@ 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;
 }
@@ -643,12 +660,13 @@ 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_protected(kn);
 	if (!on) {
-		mutex_unlock(&kernfs_open_file_mutex);
+		mutex_unlock(mutex);
 		return;
 	}
 
@@ -660,7 +678,7 @@ 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)
@@ -802,7 +820,7 @@ 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) {
 		/*
@@ -819,11 +837,12 @@ 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);
@@ -838,6 +857,7 @@ 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;
@@ -853,10 +873,10 @@ 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_check_open_node_protected(kn);
 	if (!on) {
-		mutex_unlock(&kernfs_open_file_mutex);
+		mutex_unlock(mutex);
 		return;
 	}
 
@@ -870,7 +890,7 @@ 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