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

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Date:	Thu, 29 Jul 2010 09:34:20 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] cred - synchronize rcu before releasing cred

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> It is perfectly legal for an RCU callback to invoke call_rcu().  However,
> this should be used -only- to wait for RCU readers.  If there are no
> RCU readers, the callback might be re-invoked in very short order,
> expecially on UP systems.
> 
> Or am I misunderstanding what you mean by "require call_rcu() to be
> able to cope iwth requeueing"?

I mean for call_rcu() to be called on an object that's already been
call_rcu()'d but not yet processed.

For example if struct cred gets its usage count reduced to 0, __put_cred()
will call_rcu() it, but what happens if someone comes along and resurrects it
by increasing its usage count again?  And what happens if the usage count is
reduced back to zero and __put_cred() calls call_rcu() again before
put_cred_rcu() has a chance to run?

David
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