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Subject: Re: [PATCH 1/4] kernfs: switch kernfs to use an rwsem
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        l Viro <viro@ZenIV.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Sun, 07 Jun 2020 16:40:33 +0800
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Hi Greg,

On Mon, 2020-05-25 at 13:47 +0800, Ian Kent wrote:
> @@ -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);
> +	down_read(&kernfs_rwsem);
>  	kernfs_refresh_inode(kn, inode);
> -	mutex_unlock(&kernfs_mutex);
> +	up_read(&kernfs_rwsem);
>  
>  	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);
> +	down_read(&kernfs_rwsem);
>  	kernfs_refresh_inode(kn, inode);
> -	mutex_unlock(&kernfs_mutex);
> +	up_read(&kernfs_rwsem);
>  
>  	return generic_permission(inode, mask);
>  }

I changed these from a write lock to a read lock late in the
development.

But kernfs_refresh_inode() modifies the inode so I think I should
have taken the inode lock as well as taking the read lock.

I'll look again but a second opinion (anyone) would be welcome.

Ian