[PATCH 1/2] kernfs: replace the mutex in kernfs_iop_permission with a rwlock

[Fox Chen <foxhlchen@gmail.com>]

A big global mutex in kernfs_iop_permission will significanly drag
system performance when processes concurrently open files
on kernfs in Big machines(with >= 16 cpu cores).

This patch replace the big mutex with a rwlock specifically for
protecting kernfs_node->iattribute. So that kernfs_iop_permission
can perform concurrently.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
 fs/kernfs/inode.c      | 16 ++++++++--------
 include/linux/kernfs.h |  1 +
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index fc2469a20fed..c8c2ea669e6d 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -106,9 +106,9 @@ int kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
 {
 	int ret;
 
-	mutex_lock(&kernfs_mutex);
+	write_lock(&kn->iattr_rwlock);
 	ret = __kernfs_setattr(kn, iattr);
-	mutex_unlock(&kernfs_mutex);
+	write_unlock(&kn->iattr_rwlock);
 	return ret;
 }
 
@@ -121,7 +121,7 @@ int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr)
 	if (!kn)
 		return -EINVAL;
 
-	mutex_lock(&kernfs_mutex);
+	write_lock(&kn->iattr_rwlock);
 	error = setattr_prepare(dentry, iattr);
 	if (error)
 		goto out;
@@ -134,7 +134,7 @@ int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr)
 	setattr_copy(inode, iattr);
 
 out:
-	mutex_unlock(&kernfs_mutex);
+	write_unlock(&kn->iattr_rwlock);
 	return error;
 }
 
@@ -189,9 +189,9 @@ int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
 	struct inode *inode = d_inode(path->dentry);
 	struct kernfs_node *kn = inode->i_private;
 
-	mutex_lock(&kernfs_mutex);
+	read_lock(&kn->iattr_rwlock);
 	kernfs_refresh_inode(kn, inode);
-	mutex_unlock(&kernfs_mutex);
+	read_unlock(&kn->iattr_rwlock);
 
 	generic_fillattr(inode, stat);
 	return 0;
@@ -281,9 +281,9 @@ int kernfs_iop_permission(struct inode *inode, int mask)
 
 	kn = inode->i_private;
 
-	mutex_lock(&kernfs_mutex);
+	read_lock(&kn->iattr_rwlock);
 	kernfs_refresh_inode(kn, inode);
-	mutex_unlock(&kernfs_mutex);
+	read_unlock(&kn->iattr_rwlock);
 
 	return generic_permission(inode, mask);
 }
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 89f6a4214a70..545cdb39b34b 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -156,6 +156,7 @@ struct kernfs_node {
 	unsigned short		flags;
 	umode_t			mode;
 	struct kernfs_iattrs	*iattr;
+	rwlock_t		iattr_rwlock;
 };
 
 /*
-- 
2.29.2