linux-kernel - Re: [PATCH 5/7] autofs4 - fix direct mount pending expire race

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Date:	Fri, 18 Jul 2008 16:08:04 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ian Kent <raven@...maw.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	autofs mailing list <autofs@...ux.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 5/7] autofs4 - fix direct mount pending expire race

Ian Kent <raven@...maw.net> writes:

> For direct and offset type mounts that are covered by another mount
> we cannot check the AUTOFS_INF_EXPIRING flag during a path walk
> which leads to lookups walking into an expiring mount while it is
> being expired.
>
> For example, for the direct multi-mount map entry with a couple of
> offsets:
>
> /race/mm1  /      <server1>:/<path1>
>            /om1   <server2>:/<path2>
>            /om2   <server1>:/<path3>
>
> an autofs trigger mount is mounted on /race/mm1 and when accessed
> it is over mounted and trigger mounts made for /race/mm1/om1 and
> /race/mm1/om2. So it isn't possible for path walks to see the
> expiring flag at all and they happily walk into the file system
> while it is expiring.
>
> When expiring these mounts follow_down() must stop at the autofs
> mount and all processes must block in the ->follow_link() method
> (except the daemon) until the expire is complete. This is done by
> decrementing the d_mounted field of the autofs trigger mount root
> dentry until the expire is completed. In ->follow_link() all
> processes wait on the expire and the mount following is completed
> for the daemon until the expire is complete.
>
> Signed-off-by: Ian Kent <raven@...maw.net>
>
> ---
>
>  fs/autofs4/autofs_i.h |    3 ++
>  fs/autofs4/expire.c   |   16 +++++++++--
>  fs/autofs4/root.c     |   72 +++++++++++++++++++++++++++++++++----------------
>  3 files changed, 65 insertions(+), 26 deletions(-)
>
>
> diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
> index 5d90ed3..4b40cbc 100644
> --- a/fs/autofs4/autofs_i.h
> +++ b/fs/autofs4/autofs_i.h
> @@ -52,6 +52,8 @@ struct autofs_info {
>  
>  	int		flags;
>  
> +	struct completion expire_complete;
> +
>  	struct list_head active;
>  	struct list_head expiring;
>  
> @@ -69,6 +71,7 @@ struct autofs_info {
>  };
>  
>  #define AUTOFS_INF_EXPIRING	(1<<0) /* dentry is in the process of expiring */
> +#define AUTOFS_INF_MOUNTPOINT	(1<<1) /* mountpoint status for direct expire */
>  
>  struct autofs_wait_queue {
>  	wait_queue_head_t queue;
> diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
> index 19f5bea..705b9f0 100644
> --- a/fs/autofs4/expire.c
> +++ b/fs/autofs4/expire.c
> @@ -259,13 +259,15 @@ static struct dentry *autofs4_expire_direct(struct super_block *sb,
>  	now = jiffies;
>  	timeout = sbi->exp_timeout;
>  
> -	/* Lock the tree as we must expire as a whole */
>  	spin_lock(&sbi->fs_lock);
>  	if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
>  		struct autofs_info *ino = autofs4_dentry_ino(root);
> -
> -		/* Set this flag early to catch sys_chdir and the like */
> +		if (d_mountpoint(root)) {
> +			ino->flags |= AUTOFS_INF_MOUNTPOINT;
> +			root->d_mounted--;
> +		}

This makes me uneasy.  This should take d_mounted to zero.  Then, when
the daemon actually does the unmount, won't the d_mounted drop below
zero?  Following calls to d_mountpoint will return a negative value, but
everyone treats it as a boolean, so it will evaluate to true for a brief
time.  Or did I miss something?

Cheers,

Jeff
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