From: Peter Zijlstra <peterz@infradead.org> To: David Howells <dhowells@redhat.com> Cc: Jann Horn <jannh@google.com>, Greg KH <gregkh@linuxfoundation.org>, Al Viro <viro@zeniv.linux.org.uk>, raven@themaw.net, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, linux-block@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module <linux-security-module@vger.kernel.org>, kernel list <linux-kernel@vger.kernel.org>, Kees Cook <keescook@chromium.org>, Kernel Hardening <kernel-hardening@lists.openwall.com> Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer Date: Fri, 31 May 2019 18:44:44 +0200 [thread overview] Message-ID: <20190531164444.GD2606@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <606.1559312412@warthog.procyon.org.uk> On Fri, May 31, 2019 at 03:20:12PM +0100, David Howells wrote: > Peter Zijlstra <peterz@infradead.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.
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org> To: David Howells <dhowells@redhat.com> Cc: Jann Horn <jannh@google.com>, Greg KH <gregkh@linuxfoundation.org>, Al Viro <viro@zeniv.linux.org.uk>, raven@themaw.net, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, linux-block@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module <linux-security-module@vger.kernel.org>, kernel list <linux-kernel@vger.kernel.org>, Kees Cook <keescook@chromium.org>, Kernel Hardening <kernel-hardening@lists.openwall.com> Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer Date: Fri, 31 May 2019 16:44:44 +0000 [thread overview] Message-ID: <20190531164444.GD2606@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <606.1559312412@warthog.procyon.org.uk> On Fri, May 31, 2019 at 03:20:12PM +0100, David Howells wrote: > Peter Zijlstra <peterz@infradead.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.
next prev parent reply other threads:[~2019-05-31 16:45 UTC|newest] Thread overview: 131+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-05-28 16:01 [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications David Howells 2019-05-28 16:01 ` David Howells 2019-05-28 16:01 ` [PATCH 1/7] General notification queue with user mmap()'able ring buffer David Howells 2019-05-28 16:01 ` David Howells 2019-05-28 16:26 ` Greg KH 2019-05-28 16:26 ` Greg KH 2019-05-28 17:30 ` David Howells 2019-05-28 17:30 ` David Howells 2019-05-28 23:12 ` Greg KH 2019-05-28 23:12 ` Greg KH 2019-05-29 16:06 ` David Howells 2019-05-29 16:06 ` David Howells 2019-05-29 17:46 ` Jann Horn 2019-05-29 17:46 ` Jann Horn 2019-05-29 21:02 ` David Howells 2019-05-29 21:02 ` David Howells 2019-05-31 11:14 ` Peter Zijlstra 2019-05-31 11:14 ` Peter Zijlstra 2019-05-31 12:02 ` David Howells 2019-05-31 12:02 ` David Howells 2019-05-31 13:26 ` Peter Zijlstra 2019-05-31 13:26 ` Peter Zijlstra 2019-05-31 14:20 ` David Howells 2019-05-31 14:20 ` David Howells 2019-05-31 16:44 ` Peter Zijlstra [this message] 2019-05-31 16:44 ` Peter Zijlstra 2019-05-31 17:12 ` David Howells 2019-05-31 17:12 ` David Howells 2019-06-17 16:24 ` Peter Zijlstra 2019-06-17 16:24 ` Peter Zijlstra 2019-05-29 23:09 ` Greg KH 2019-05-29 23:09 ` Greg KH 2019-05-29 23:11 ` Greg KH 2019-05-29 23:11 ` Greg KH 2019-05-30 9:50 ` Andrea Parri 2019-05-30 9:50 ` Andrea Parri 2019-05-31 8:35 ` Peter Zijlstra 2019-05-31 8:35 ` Peter Zijlstra 2019-05-31 8:47 ` Peter Zijlstra 2019-05-31 8:47 ` Peter Zijlstra 2019-05-31 12:42 ` David Howells 2019-05-31 12:42 ` David Howells 2019-05-31 14:55 ` David Howells 2019-05-31 14:55 ` David Howells 2019-05-28 19:14 ` Jann Horn 2019-05-28 19:14 ` Jann Horn 2019-05-28 22:28 ` David Howells 2019-05-28 22:28 ` David Howells 2019-05-28 23:16 ` Jann Horn 2019-05-28 23:16 ` Jann Horn 2019-05-28 16:02 ` [PATCH 2/7] keys: Add a notification facility David Howells 2019-05-28 16:02 ` David Howells 2019-05-28 16:02 ` [PATCH 3/7] vfs: Add a mount-notification facility David Howells 2019-05-28 16:02 ` David Howells 2019-05-28 20:06 ` Jann Horn 2019-05-28 20:06 ` Jann Horn 2019-05-28 23:04 ` David Howells 2019-05-28 23:04 ` David Howells 2019-05-28 23:23 ` Jann Horn 2019-05-28 23:23 ` Jann Horn 2019-05-29 11:16 ` David Howells 2019-05-29 11:16 ` David Howells 2019-05-28 23:08 ` David Howells 2019-05-28 23:08 ` David Howells 2019-05-29 10:55 ` David Howells 2019-05-29 10:55 ` David Howells 2019-05-29 11:00 ` David Howells 2019-05-29 11:00 ` David Howells 2019-05-29 15:53 ` Casey Schaufler 2019-05-29 15:53 ` Casey Schaufler 2019-05-29 16:12 ` Jann Horn 2019-05-29 16:12 ` Jann Horn 2019-05-29 17:04 ` Casey Schaufler 2019-05-29 17:04 ` Casey Schaufler 2019-06-03 16:30 ` David Howells 2019-06-03 16:30 ` David Howells 2019-05-29 17:13 ` Andy Lutomirski 2019-05-29 17:13 ` Andy Lutomirski 2019-05-29 17:46 ` Casey Schaufler 2019-05-29 17:46 ` Casey Schaufler 2019-05-29 18:11 ` Jann Horn 2019-05-29 18:11 ` Jann Horn 2019-05-29 19:28 ` Casey Schaufler 2019-05-29 19:28 ` Casey Schaufler 2019-05-29 19:47 ` Jann Horn 2019-05-29 19:47 ` Jann Horn 2019-05-29 20:50 ` Casey Schaufler 2019-05-29 20:50 ` Casey Schaufler 2019-05-29 23:12 ` Andy Lutomirski 2019-05-29 23:12 ` Andy Lutomirski 2019-05-29 23:56 ` Casey Schaufler 2019-05-29 23:56 ` Casey Schaufler 2019-05-28 16:02 ` [PATCH 4/7] vfs: Add superblock notifications David Howells 2019-05-28 16:02 ` David Howells 2019-05-28 20:27 ` Jann Horn 2019-05-28 20:27 ` Jann Horn 2019-05-29 12:58 ` David Howells 2019-05-29 12:58 ` David Howells 2019-05-29 14:16 ` Jann Horn 2019-05-29 14:16 ` Jann Horn 2019-05-28 16:02 ` [PATCH 5/7] fsinfo: Export superblock notification counter David Howells 2019-05-28 16:02 ` David Howells 2019-05-28 16:02 ` [PATCH 6/7] block: Add block layer notifications David Howells 2019-05-28 16:02 ` David Howells 2019-05-28 20:37 ` Jann Horn 2019-05-28 20:37 ` Jann Horn 2019-05-28 16:02 ` [PATCH 7/7] Add sample notification program David Howells 2019-05-28 16:02 ` David Howells 2019-05-28 23:58 ` [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications Greg KH 2019-05-28 23:58 ` Greg KH 2019-05-29 6:33 ` Amir Goldstein 2019-05-29 6:33 ` Amir Goldstein 2019-05-29 6:33 ` Amir Goldstein 2019-05-29 14:25 ` Jan Kara 2019-05-29 14:25 ` Jan Kara 2019-05-29 15:10 ` Greg KH 2019-05-29 15:10 ` Greg KH 2019-05-29 15:53 ` Amir Goldstein 2019-05-29 15:53 ` Amir Goldstein 2019-05-30 11:00 ` Jan Kara 2019-05-30 11:00 ` Jan Kara 2019-06-04 12:33 ` David Howells 2019-06-04 12:33 ` David Howells 2019-05-29 6:45 ` David Howells 2019-05-29 6:45 ` David Howells 2019-05-29 7:40 ` Amir Goldstein 2019-05-29 7:40 ` Amir Goldstein 2019-05-29 9:09 ` David Howells 2019-05-29 9:09 ` David Howells 2019-05-29 15:41 ` Casey Schaufler 2019-05-29 15:41 ` Casey Schaufler
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