linux-kernel - Re: [announce] vfs-scale git tree update

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Message-ID: <AANLkTim8rQb3mDFHg7DmbTZa20FxRGk3wJCjOkz=JG9S@mail.gmail.com>
Date:	Tue, 11 Jan 2011 20:06:41 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ian Kent <raven@...maw.net>
Cc:	aelder@....com, Nick Piggin <npiggin@...nel.dk>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [announce] vfs-scale git tree update

On Tue, Jan 11, 2011 at 7:59 PM, Ian Kent <raven@...maw.net> wrote:
>
> Isn't the parent i_mutex held during mkdir()?

Yes, but a lookup that hits in the dentry cache won't actually take
the parent mutex.

So as far as I can tell, doing the "d_add()" before setting d_op can
result in another CPU coming in and seeing the newly added dentry
before d_op has actually been initialized. Exactly because it will do
the dentry lookup without holding any mutex.

Of course, it's a very small window, so it probably doesn't matter in practice.

>> Looking at it quickly, I don't think that would matter for
>> the case at hand.  I.e., that might be safer but it doesn't
>> address the fact that these fields are getting initialized
>> multiple times.
>
> Yeah, a hangover from changes done over time.
> Not setting the dentry op in ->lookup() should fix this.

Alex, care to test just removing the d_set_d_op() call from autofs4_lookup()?

(That code is a bit scary, though - it explicitly makes it a negative
dentry with a d_instantiate(dentry, NULL), and then hides the inode
information away separately. Scary scary)

              Linus
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