linux-kernel - Re: [PATCH] nfs: lookupcache coherence bugs in WCC update path (revised)

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Date:	Thu, 12 Aug 2010 11:49:55 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	"Patrick J. LoPresti" <lopresti@...il.com>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfs: lookupcache coherence bugs in WCC update path
 (revised)

On Wed, 2010-08-11 at 22:26 -0700, Patrick J. LoPresti wrote:
> (Well, crud.  I screwed up the previous diff and was missing a
> close-curly.  This version actually compiles...)
> 
> This patch fixes some coherence bugs in the NFS "dentry lookup cache".
> 
> The NFS dentry lookup cache provides the nfs_force_lookup_revalidate()
> call to invalidate all cached dentries associated with an inode.  In
> general, the NFS client uses the ctime and mtime in the inode to detect
> when changes are made on the server.
> 
> Therefore, to maintain cache coherence, nfs_force_lookup_revalidate()
> must be called whenever the ctime or mtime of a directory inode is
> updated with fresh data from the server.  There are a few spots in
> nfs_wcc_update_inode() where this rule is violated, making it possible
> for the lookup cache to return arbitrarily stale data.
> 
> This actually bit me in practice.  I have an application where a
> negative dentry results in -ENOENT for a file that was created 30+
> minutes earlier (despite the "noac" mount option).  Unfortunately I
> cannot share my test case, but I believe the following simple patch is
> "obviously correct", and I can confirm that it fixes my issue.

This looks less than obviously correct to me. The wcc case is invoked
when the ctime/mtime/.... change is known to have occurred due to a file
creation/unlink/... from this client. It is a weak cache consistency
case.

If your client is seeing ENOENT after it created the file itself, then
the problem isn't cache coherency, but a bug in the file creation code.

Cheers
  Trond

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