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

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Message-ID: <20091211163521.GC6803@linux.vnet.ibm.com>
Date:	Fri, 11 Dec 2009 08:35:21 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 0/9] Fix various __task_cred related invalid RCU
	assumptions

On Fri, Dec 11, 2009 at 01:39:06PM +0000, David Howells wrote:
> Thomas Gleixner <tglx@...utronix.de> 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
> 
> I'm sure last time I looked, spinlock primitives implied RCU read locks.
> Maybe I was mistaken or maybe it's changed.  Whatever, good catch, Thomas!

It did indeed change with the split of the old synchronize_kernel()
primitive into the synchronize_rcu() and synchronize_sched() primitives
in 2.6.12 -- after this point, synchronize_rcu() was only guaranteed
to respect "real" rcu_read_lock()-base critical sections.  But actual
failures would not show up until PREEMPT_RCU was introduced into 2.6.25.

The -rt effort rooted out a bunch of these sorts of problems, but we
clearly missed a few.

							Thanx, Paul
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