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

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Date:	Mon, 24 Feb 2014 16:01:29 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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>,
	"J. Bruce Fields" <bfields@...ldses.org>
Subject: Re: [PATCH 08/11] vfs: Merge check_submounts_and_drop and d_invalidate

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. (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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