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Subject: [PATCH 1/2] kernfs: dont take i_lock on inode attr read
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>
Cc:     Minchan Kim <minchan@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>,
        Al Viro <viro@ZenIV.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Tue, 18 Oct 2022 10:32:42 +0800
Message-ID: <166606036215.13363.1288735296954908554.stgit@donald.themaw.net>
In-Reply-To: <166606025456.13363.3829702374064563472.stgit@donald.themaw.net>
References: <166606025456.13363.3829702374064563472.stgit@donald.themaw.net>
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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.

Signed-off-by: Ian Kent <raven@themaw.net>
---
 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;