linux-kernel - Re: [PATCH 1/2] vfs: dcache: fix deadlock in tree traversal

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Message-ID: <CA+55aFxJKQMjGm00kBpj30twj1w60Mx4C0mXTJtHiJPOpu-iMA@mail.gmail.com>
Date:	Tue, 18 Sep 2012 08:56:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	Trond.Myklebust@...app.com
Subject: Re: [PATCH 1/2] vfs: dcache: fix deadlock in tree traversal

On Tue, Sep 18, 2012 at 7:53 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
>
> Yes, with the test cases that IBM were using it is DCACHE_DISCONNECTED
> case that triggers the double-lock.  Trond was misusing
> DCACHE_DISCONNECTED and this made the failure in try_to_ascend() much
> more likely (and bogus).  But there is a case, which is triggered rarely
> if ever, when try_to_ascend() failure with rename_lock held is perfectly
> valid.

Ok. The whole DCACHE_DISCONNECTED logic there is clearly bogus and
results in endless loops if that case then ever triggers, but you fix
that in the second patch.

HOWEVER. Why introduce that new DCACHE_KILLED flag at all? Wouldn't it
be much better to just check whether the dentry is hashed instead of
introducing a new flag for this case? Couldn't we just check for
"d_unhashed()"?

Anyway, I don't hate the 2/2 thing, but these are clearly related, and
I'd like to understand why we had that odd DCACHE_DISCONNECTED test in
the first place when there *seems* to be more straightforward
approaches to it?

              Linus
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