linux-kernel - Re: [PATCH 01/46] Revert "fs: use RCU read side protection in d

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Message-ID: <20101208011656.GD29333@dastard>
Date:	Wed, 8 Dec 2010 12:16:56 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/46] Revert "fs: use RCU read side protection in
 d_validate"

On Sat, Nov 27, 2010 at 08:56:03PM +1100, Nick Piggin wrote:
> This reverts commit 3825bdb7ed920845961f32f364454bee5f469abb.
> 
> Patch is broken, you can't dget() without holding any locks!

I believe you can - for the same reasons we can take a reference to
an inode without holding the inode_lock. That is, as long as the
caller already holds an active reference to the dentry,
dget() can be used to take another reference without needing the
dcache_lock.

Such usage appears to be described in the comment above dget() and
there's a BUG_ON() in dget() to catch callers that don't already
have an active reference. An example of a valid unlocked dget():
d_alloc() does an unlocked dget() to take a reference to the parent
dentry which we already are guaranteed to have a reference to.

As to d_validate() - it depends on the caller behaviour as to
whether the unlocked dget() is valid or not.  From a cursory check
of the NCP and SMB readdir caches, both appear to hold an active
reference to the dentry it is passing to d_validate(). If that is
the case then there is nothing wrong with the way d_validate uses
dget(). Can someone with more SMB/NCP expertise than me validate the
use of cached dentries?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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