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Date: Fri, 31 May 2019 18:44:44 +0200
From: Peter Zijlstra <>
To: David Howells <>
Cc: Jann Horn <>, Greg KH <>,
	Al Viro <>,,
	linux-fsdevel <>,
	Linux API <>,,,
	linux-security-module <>,
	kernel list <>,
	Kees Cook <>,
	Kernel Hardening <>
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able
 ring buffer

On Fri, May 31, 2019 at 03:20:12PM +0100, David Howells wrote:
> Peter Zijlstra <> wrote:

> > (and it has already been established that refcount_t doesn't work for
> > usage count scenarios)
> ?
> Does that mean struct kref doesn't either?

Indeed, since kref is just a pointless wrapper around refcount_t it does
not either.

The main distinction between a reference count and a usage count is that
0 means different things. For a refcount 0 means dead. For a usage count
0 is merely unused but valid.

Incrementing a 0 refcount is a serious bug -- use-after-free (and hence
refcount_t will refuse this and splat), for a usage count this is no

Now, it is sort-of possible to merge the two, by basically stating
something like: usage = refcount - 1. But that can get tricky and people
have not really liked the result much for the few times I tried.

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