linux-kernel - Re: [PATCH 04/11] vfs: check unlinked ancestors before mount

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Message-ID: <20130905132325.GQ13318@ZenIV.linux.org.uk>
Date:	Thu, 5 Sep 2013 14:23:25 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>,
	David Howells <dhowells@...hat.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 04/11] vfs: check unlinked ancestors before mount

On Thu, Sep 05, 2013 at 02:39:11PM +0200, Miklos Szeredi wrote:
> +static bool __has_unlinked_ancestor(struct dentry *dentry)
> +{
> +	struct dentry *this;
> +
> +	for (this = dentry; !IS_ROOT(this); this = this->d_parent) {
> +		int is_unhashed;
> +
> +		/* Need exclusion wrt. check_submounts_and_drop() */
> +		spin_lock(&this->d_lock);
> +		is_unhashed = d_unhashed(this);
> +		spin_unlock(&this->d_lock);
> +
> +		if (is_unhashed)
> +			return true;
> +	}
> +	return false;
> +}

I still don't get it; why do you need to bother with early setting of
DCACHE_MOUNTED?

You are grabbing rename_lock for write in d_set_mounted().  What kind of races
with check for submounts are you worried about?  d_walk() will rescan
everything if something grabs rename_lock for write while it had been running,
so just fold the "have nothing in d_subdir" case of check_submounts_and_drop()
into d_walk() and be done with that...  What's the problem with such
variant?  AFAICS, all you need to care about is d_set_mounted() not getting
between the scan for submounts and actual __d_drop() and your "finish"
callback is called only after d_walk() having grabbed d_lock *and* rechecked
rename_lock.
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