linux-kernel - Re: [PATCH 12/22] elevate write count files are open()ed

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Date:	Mon, 12 Feb 2007 21:11:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Hansen <hansendc@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, hch@...radead.org
Subject: Re: [PATCH 12/22] elevate write count files are open()ed

On Fri, 09 Feb 2007 14:53:37 -0800 Dave Hansen <hansendc@...ibm.com> wrote:

> diff -puN fs/file_table.c~14-24-tricky-elevate-write-count-files-are-open-ed fs/file_table.c
> --- lxc/fs/file_table.c~14-24-tricky-elevate-write-count-files-are-open-ed	2007-02-09 14:26:54.000000000 -0800
> +++ lxc-dave/fs/file_table.c	2007-02-09 14:26:54.000000000 -0800
> @@ -209,8 +209,11 @@ void fastcall __fput(struct file *file)
>  	if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL))
>  		cdev_put(inode->i_cdev);
>  	fops_put(file->f_op);
> -	if (file->f_mode & FMODE_WRITE)
> +	if (file->f_mode & FMODE_WRITE) {
>  		put_write_access(inode);
> +		if(!special_file(inode->i_mode))
> +			mnt_drop_write(mnt);
> +	}
>  	put_pid(file->f_owner.pid);
>  	put_user_ns(file->f_owner.user_ns);
>  	file_kill(file);
> diff -puN fs/namei.c~14-24-tricky-elevate-write-count-files-are-open-ed fs/namei.c
> --- lxc/fs/namei.c~14-24-tricky-elevate-write-count-files-are-open-ed	2007-02-09 14:26:54.000000000 -0800
> +++ lxc-dave/fs/namei.c	2007-02-09 14:26:54.000000000 -0800
> @@ -1548,8 +1548,17 @@ int may_open(struct nameidata *nd, int a
>  			return -EACCES;
>  
>  		flag &= ~O_TRUNC;
> -	} else if (IS_RDONLY(inode) && (flag & FMODE_WRITE))
> -		return -EROFS;
> +	} else if (flag & FMODE_WRITE) {
> +		/*
> +		 * effectively: !special_file()
> +		 * balanced by __fput()
> +		 */
> +		error = mnt_want_write(nd->mnt);
> +		if (error)
> +			return error;
> +		if (IS_RDONLY(inode))
> +			return -EROFS;
> +	}

yipes.  A new mount-wide spin_lock/unlock for each for-writing open() and close().
Can we have a microbenchmark on this please?

Are you sure that fget_light() and fput_light() don't accidentally bypass this
new logic?
-
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