linux-kernel - Re: Linux 2.6.26-rc4

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Message-ID: <20080603173029.GD28946@ZenIV.linux.org.uk>
Date:	Tue, 3 Jun 2008 18:30:30 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Ian Kent <raven@...maw.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>, jesper@...gh.cc,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Linux 2.6.26-rc4

On Wed, Jun 04, 2008 at 01:13:08AM +0800, Ian Kent wrote:

> "What happens is that during an expire the situation can arise
> that a directory is removed and another lookup is done before
> the expire issues a completion status to the kernel module.
> In this case, since the the lookup gets a new dentry, it doesn't
> know that there is an expire in progress and when it posts its
> mount request, matches the existing expire request and waits
> for its completion. ENOENT is then returned to user space
> from lookup (as the dentry passed in is now unhashed) without
> having performed the mount request.
> 
> The solution used here is to keep track of dentrys in this
> unhashed state and reuse them, if possible, in order to
> preserve the flags. Additionally, this infrastructure will
> provide the framework for the reintroduction of caching
> of mount fails removed earlier in development."
> 
> I wasn't able to do an acceptable re-implementation of the negative
> caching we had in 2.4 with this framework, so just ignore the last
> sentence in the above description. 

> Unfortunately no, but I thought that once the dentry became unhashed
> (aka ->rmdir() or ->unlink()) it was invisible to the dcache. But, of
> course there may be descriptors open on the dentry, which I think is the
> problem that's being pointed out.
 
... or we could have had a pending mount(2) sitting there with a reference
to mountpoint-to-be...

> Yes, that would be ideal but the reason we arrived here is that, because
> we must release the directory mutex before calling back to the daemon
> (the heart of the problem, actually having to drop the mutex) to perform
> the mount, we can get a deadlock. The cause of the problem was that for
> "create" like operations the mutex is held for ->lookup() and
> ->revalidate() but for a "path walks" the mutex is only held for
> ->lookup(), so if the mutex is held when we're in ->revalidate(), we
> could never be sure that we where the code path that acquired it.
> 
> Sorry, this last bit is unclear.
> I'll need to work a bit harder on the explanation if you're interested
> in checking further.

I am.

Oh, well...  Looks like RTFS time for me for now...  Additional parts of
braindump would be appreciated - the last time I've seriously looked at
autofs4 internal had been ~2005 or so ;-/
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