linux-kernel - Re: [patch 0/9] Fix various __task_cred related invalid RCU assumptions

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Message-ID: <alpine.LFD.2.00.0912091914540.3560@localhost.localdomain>
Date:	Wed, 9 Dec 2009 19:15:52 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 0/9] Fix various __task_cred related invalid RCU
 assumptions



On Wed, 9 Dec 2009, Paul E. McKenney wrote:

> On Thu, Dec 10, 2009 at 12:52:46AM -0000, Thomas Gleixner wrote:
> > While auditing the read_lock(&tasklist_lock) sites for a possible
> > conversion to rcu-read_lock() I stumbled over an unprotected user of
> > __task_cred in kernel/sys.c
> > 
> > That caused me to audit all the __task_cred usage sites except in
> > kernel/exit.c.
> > 
> > Most of the usage sites are correct, but some of them trip over
> > invalid assumptions about the protection which is given by RCU.
> > 
> > - spinlocked/preempt_disabled regions are equivalent to rcu_read_lock():
> > 
> >    That's wrong. RCU does not guarantee that. 
> > 
> >    It has been that way due to implementation details and it still is
> >    valid for CONFIG_TREE_PREEMPT_RCU=n, but there is no guarantee that
> >    this will be the case forever.
> 
> To back this up, item #2 from Documentation/RCU/checklist.txt says:

Hmm. This seems to be a difference that the tree-RCU things introduced, 
no? I wonder if we have other areas where we just knew that a spinlock 
would make an rcu read-lock unnecessary (which used to be true..)

		Linus
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