linux-kernel - Re: [PATCH 08/11] vfs: Merge check_submounts_and_drop and d

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Message-ID: <20140225151349.GA19981@fieldses.org>
Date:	Tue, 25 Feb 2014 10:13:49 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Karel Zak <kzak@...hat.com>
Subject: Re: [PATCH 08/11] vfs: Merge check_submounts_and_drop and
 d_invalidate

On Mon, Feb 24, 2014 at 04:01:29PM -0800, Eric W. Biederman wrote:
> Miklos Szeredi <miklos@...redi.hu> writes:
> 
> > On Sat, Feb 15, 2014 at 01:39:22PM -0800, Eric W. Biederman wrote:
> >> 
> >> Now that d_invalidate is the only caller of check_submounts_and_drop,
> >> expand check_submounts_and_drop inline in d_invalidate.
> >> 
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> >> ---
> >>  fs/dcache.c            |   55 +++++++++++++++++++----------------------------
> >>  include/linux/dcache.h |    1 -
> >>  2 files changed, 22 insertions(+), 34 deletions(-)
> >> 
> >> diff --git a/fs/dcache.c b/fs/dcache.c
> >> index 27585b1dd6f1..5b41205cbf33 100644
> >> --- a/fs/dcache.c
> >> +++ b/fs/dcache.c
> 
> >> -int check_submounts_and_drop(struct dentry *dentry)
> >> +int d_invalidate(struct dentry *dentry)
> >>  {
> >>  	int ret = 0;
> >>  
> >> +	/*
> >> +	 * If it's already been dropped, return OK.
> >> +	 */
> >> +	spin_lock(&dentry->d_lock);
> >> +	if (d_unhashed(dentry)) {
> >> +		spin_unlock(&dentry->d_lock);
> >> +		return 0;
> >> +	}
> >> +	spin_unlock(&dentry->d_lock);
> >> +
> >>  	/* Negative dentries can be dropped without further checks */
> >>  	if (!dentry->d_inode) {
> >>  		d_drop(dentry);
> >
> >
> > You can optimize this by including the negative check within the above d_locked
> > region and calling __d_drop() instead.
> 
> For this patch just moving the code and not changing it is the corret
> thing to do because it helps with review and understanding the code.
> 
> There are two ways I could see going with optimizing the preamble.
> Simply dropping the d_lock from around the d_unhashed test as a pointer
> dereference should be atomic, and the test is racy against
> d_materialise_unique.

Could you explain?  What's the race, and what are the consequences?

--b.

> (We don't always hold the parent
> directories inode mutex when d_invalidate is called).  So the d_lock
> buys us very little.  Alternatively we could move the work into the
> d_walk callbacks.
> 
> That kind of optimization deserves it's own patch that can be reviewed
> independently.
> 
> Eric
> --
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