linux-kernel - Re: [patch 01/28] fs: d

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Message-ID: <20101119050507.GC3284@amd>
Date:	Fri, 19 Nov 2010 16:05:07 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	David Miller <davem@...emloft.net>
Cc:	npiggin@...nel.dk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 01/28] fs: d_validate fixes

On Thu, Nov 18, 2010 at 12:59:13PM -0800, David Miller wrote:
> From: David Miller <davem@...emloft.net>
> Date: Thu, 18 Nov 2010 12:51:23 -0800 (PST)
> 
> > From: Nick Piggin <npiggin@...nel.dk>
> > Date: Wed, 17 Nov 2010 01:09:01 +1100
> > 
> >> d_validate has been broken for a long time.
> >> 
> >> kmem_ptr_validate does not guarantee that a pointer can be dereferenced
> >> if it can go away at any time. Even rcu_read_lock doesn't help, because
> >> the pointer might be queued in RCU callbacks but not executed yet.
> >> 
> >> So the parent cannot be checked, nor the name hashed. The dentry pointer
> >> can not be touched until it can be verified under lock. Hashing simply
> >> cannot be used.
> >> 
> >> Instead, verify the parent/child relationship by traversing parent's
> >> d_child list. It's slow, but only ncpfs and the destaged smbfs care
> >> about it, at this point.
> >> 
> >> Signed-off-by: Nick Piggin <npiggin@...nel.dk>
> > 
> > This won't apply because is conflicts with Christoph Hellwig's
> > RCU conversion of d_validate().
> > 
> > Which is a change that went in more than a month ago.
> 
> In fact the conflicts of your patch set are even more pervasive, since
> all dcache hash traversals are essentially RCU protected instead of
> dcache_lock protected right now.

Not sure what you mean there. The patches are against upstream+revert of
the last d_validate patch.

dcache_lock splitup of this series is to split the lock out of all the
other paths, and importantly allow d_lock to protect the complete
dcache state of the dentry.

Next 2 steps (that depend on this series but not on each other) are
fine grained locking of the split locks, and rcu-walk. rcu-walk is what
I called store-free path walking, because we extend RCU not only to the
hash lookup but the entire path walk.

I'll get all that out when I get a bit of time to work on it again.

Thanks,
Nick
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