linux-kernel - Re: [PATCH] cred - synchronize rcu before releasing cred

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Date:	Tue, 27 Jul 2010 09:16:16 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cred - synchronize rcu before releasing cred

On Tue, Jul 27, 2010 at 8:50 AM, Jiri Olsa <jolsa@...hat.com> wrote:
>
> got no objections on linux-security-module and acked by David.
> Noone pick it, so got advice to send this directly to you.

The patch seems fundamentally buggy.

The whole patch seems to be based on "nobody can ever use
get_cred/put_cred, because concurrent use will then trigger the
BUG_ON() in __put_cred()".

But that's a bug in general, not in this particular usage that isn't
all that different from other uses. So rather than just remove the
code that uses the refcounting, we should either:

 - FIX the damn ref-counting so that it works without bugging out

or

 - remove the broken functions entirely.

In other words - why are we working around what looks like a bug,
rather than fixing the bug itself?

In particular, the code you remove seems to be basically _identical_
to get_task_cred(). So if the code you remove is buggy, then so is any
use of get_task_cred() - no?

So please explain why get_task_cred() is ok, but the particular use of
get_cred/put_cred that you removed is not.

Hmm? What am I missing?

                                   Linus
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