linux-kernel - Re: [PATCH 09/30] r/o bind mounts: drop write during emergency remount

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Message-Id: <E1JR8rj-0003ul-Di@pomaz-ex.szeredi.hu>
Date:	Mon, 18 Feb 2008 17:29:15 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	haveblue@...ibm.com
CC:	linux-kernel@...r.kernel.org, hch@....de, miklos@...redi.hu,
	akpm@...l.org, haveblue@...ibm.com
Subject: Re: [PATCH 09/30] r/o bind mounts: drop write during emergency remount

> The emergency remount code forcibly removes FMODE_WRITE from
> filps.  The r/o bind mount code notices that this was done
> without a proper mnt_drop_write() and properly gives a
> warning.
> 
> This patch does a mnt_drop_write() to keep everything
> balanced.
> 	
> Acked-by: Al Viro <viro@...IV.linux.org.uk>
> Signed-off-by: Christoph Hellwig <hch@...radead.org>
> Signed-off-by: Dave Hansen <haveblue@...ibm.com>
> ---
> 
>  linux-2.6.git-dave/fs/super.c |   20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff -puN fs/super.c~robind-sysrq-fix fs/super.c
> --- linux-2.6.git/fs/super.c~robind-sysrq-fix	2008-02-15 13:25:46.000000000 -0800
> +++ linux-2.6.git-dave/fs/super.c	2008-02-15 13:25:46.000000000 -0800
> @@ -37,6 +37,7 @@
>  #include <linux/idr.h>
>  #include <linux/kobject.h>
>  #include <linux/mutex.h>
> +#include <linux/file.h>
>  #include <asm/uaccess.h>
>  
>  
> @@ -567,10 +568,25 @@ static void mark_files_ro(struct super_b
>  {
>  	struct file *f;
>  
> +retry:
>  	file_list_lock();
>  	list_for_each_entry(f, &sb->s_files, f_u.fu_list) {
> -		if (S_ISREG(f->f_path.dentry->d_inode->i_mode) && file_count(f))
> -			f->f_mode &= ~FMODE_WRITE;
> +		struct vfsmount *mnt;
> +		if (!S_ISREG(f->f_path.dentry->d_inode->i_mode))
> +		       continue;
> +		if (!file_count(f))
> +			continue;
> +		if (!(f->f_mode & FMODE_WRITE))
> +			continue;
> +		f->f_mode &= ~FMODE_WRITE;
> +		mnt = f->f_path.mnt;

AFAICS an mntget() is needed here to make sure mnt doesn't go away
after releasing file_list_lock.

Miklos

> +		file_list_unlock();
> +		/*
> +		 * This can sleep, so we can't hold
> +		 * the file_list_lock() spinlock.
> +		 */
> +		mnt_drop_write(mnt);
> +		goto retry;
>  	}
>  	file_list_unlock();
>  }
> _
> 
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