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

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Message-ID: <20100729060004.GD2447@linux.vnet.ibm.com>
Date:	Wed, 28 Jul 2010 23:00:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	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

On Wed, Jul 28, 2010 at 01:47:06PM +0100, David Howells wrote:
> David Howells <dhowells@...hat.com> wrote:
> 
> > Yeah.  I think there are three alternatives:
> 
> There's a fourth alternative too:
> 
>  (4) I could try and make it so that if the RCU cleanup routine sees it with a
>      non-zero usage count, then it just ignores it.  This, however, would
>      require call_rcu() to be able to cope with requeueing.

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"?

							Thanx, Paul
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