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Date:   Tue, 7 May 2019 20:55:03 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     yangerkun <yangerkun@huawei.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        yi.zhang@huawei.com, houtao1@huawei.com, miaoxie@huawei.com
Subject: Re: system panic while dentry reference count overflow
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On Tue, May 07, 2019 at 12:23:23PM -0700, Linus Torvalds wrote:
> On Tue, May 7, 2019 at 12:16 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Negative ->d_lockref.count are used for "lockref is dead"...
> 
> We can change that to just -1, can't we? It's equally easy to test for.

	Provided that lockref.c is updated accordingly (look at e.g.
lockref_get_not_zero()).

> Those aren't supposed to be incremented anyway, which is the whole point.
> 
> But we could do what the page refs also did: consider refcounts in the
> "small negative range" to be very special, because they are either
> critically close to an overflow, or they are actually a sign of a
> fatal underflow due to some bug. And make one of those be the dead
> marker.

	lockref_get_not_zero() hitting dead dentry is not abnormal,
so we'd better not complain in such case...  BTW, wouldn't that WARN_ON()
in dget() belong in lockref_get()?