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

[Fox Chen]

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 <foxhlc...@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