Date: Fri, 31 May 2019 18:44:44 +0200 From: Peter Zijlstra <peterz@...radead.org> To: David Howells <dhowells@...hat.com> Cc: Jann Horn <jannh@...gle.com>, Greg KH <gregkh@...uxfoundation.org>, Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net, linux-fsdevel <linux-fsdevel@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, linux-block@...r.kernel.org, keyrings@...r.kernel.org, linux-security-module <linux-security-module@...r.kernel.org>, kernel list <linux-kernel@...r.kernel.org>, Kees Cook <keescook@...omium.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> 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 <peterz@...radead.org> 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 problem. 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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