[SRU][J][PATCH 2/6] kernfs: dont take i_lock on inode attr read

[Ghadi Elie Rahme]

From: Ian Kent <raven at themaw.net>

The kernfs write lock is held when the kernfs node inode attributes
are updated. Therefore, when either kernfs_iop_getattr() or
kernfs_iop_permission() are called the kernfs node inode attributes
won't change.

Consequently concurrent kernfs_refresh_inode() calls always copy the
same values from the kernfs node.

So there's no need to take the inode i_lock to get consistent values
for generic_fillattr() and generic_permission(), the kernfs read lock
is sufficient.

Cc: Tejun Heo <tj at kernel.org>
Signed-off-by: Ian Kent <raven at themaw.net>
Link: https://lore.kernel.org/r/166606036215.13363.1288735296954908554.stgit@donald.themaw.net
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(backported from commit aa1d058d48f292aa138e33ad12b7b4d18b5407cd)
Signed-off-by: Ghadi Elie Rahme <ghadi.rahme at canonical.com>
---
 fs/kernfs/inode.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index 3d783d80f5da..74f3453f4639 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -190,10 +190,8 @@ int kernfs_iop_getattr(struct user_namespace *mnt_userns,
 	struct kernfs_root *root = kernfs_root(kn);
 
 	down_read(&root->kernfs_rwsem);
-	spin_lock(&inode->i_lock);
 	kernfs_refresh_inode(kn, inode);
 	generic_fillattr(&init_user_ns, inode, stat);
-	spin_unlock(&inode->i_lock);
 	up_read(&root->kernfs_rwsem);
 
 	return 0;
@@ -288,10 +286,8 @@ int kernfs_iop_permission(struct user_namespace *mnt_userns,
 	root = kernfs_root(kn);
 
 	down_read(&root->kernfs_rwsem);
-	spin_lock(&inode->i_lock);
 	kernfs_refresh_inode(kn, inode);
 	ret = generic_permission(&init_user_ns, inode, mask);
-	spin_unlock(&inode->i_lock);
 	up_read(&root->kernfs_rwsem);
 
 	return ret;
-- 
2.34.1